Process for synthesizing carbon coated sodium vanadium phosphate and product thereof

Abstract

A process for synthesizing carbon coated sodium vanadium phosphate and product thereof is provided. The process involves producing in-situ carbon coated sodium vanadium phosphate (Na3V2(PO4)3) via additive-assisted high energy milling. The process offers significant advantages, including reduced synthesis time, energy efficiency, scalability, and cost-effectiveness. The resultant in-situ carbon coated sodium vanadium phosphate exhibits uniform particle size and improved conductivity and stability, making it suitable for various energy storage applications, such as sodium-ion batteries, sodium-ion symmetric cells, and hybrid supercapacitors. The process provides a commercially viable route for producing high- performance electrode materials with superior cycling stability and specific energy.

Description

PROCESS FOR SYNTHESIZING CARBON COATED SODIUM VANADIUMPHOSPHATE AND PRODUCT THEREOFFIEED OF INVENTION

[0001] The present disclosure relates generally to the field of material science and electrochemistry, specifically to the synthesis of advanced electrode materials for energy storage devices. In particular, the present disclosure pertains to a process for synthesizing in- i tu carbon coated sodium vanadium phosphate and product thereof.BACKGROUND

[0002] Electrochemical energy storage devices, particularly rechargeable batteries are considered as the most preferred option to store the energy compared to other storage devices due to their high specific energy and power density. For example: Lithium ion batteries (LIBs) are being used in electric vehicles (EVs) and grid energy storage since its successful commercialization by Sony in 1990 [Hwang et al., 2017, Chem. Soc. Rev. 46, 3529-3614], LIB possesses high specific energy, long cycle life and design flexibility, which make them suitable for different applications.

[0003] However, limited Lithium resources, high cost and poor low-temperature storage performance are the key issues associated with LIBs and are the limiting factors for their applications. For example, the cost of Lithium makes the LIBs not suitable for large scale grid storage. Sodium ion batteries (SIBs) are considered as potential alternative to LIBs due to abundance of sodium resources (being one of the primary elements in sea-water), low cost and on-par specific energy. In addition, similar working principle, such as the reversible shuttling of sodium ions between electrode materials through electrolyte to that of LIBs, add to its credentials [C. Delmas, “Sodium and Sodium-Ion Batteries: 50 Years of research”, Adv. Energy Mater. (2018) 1703137],

[0004] Further, the structural and thermal stability of the sodium containing electrode materials compared to their lithium counterpart, make them suitable for variety of applications. The intercalation chemistry being same for both lithium and sodium, allows using similar compounds for SIBs. However, the larger size of sodium ion compared to the lithium ion affects the phase stability, transport properties, and solid electrolyte interphase (SEI) formation [B. Dunn, H. Kamath, J. M. Tarascon, “Electrical Energy Storage for the Grid: A battery of choices”, Science (2011) 334, 928-935],

[0005] Like LIBs, different electrode materials based on their crystal structure, such as layered type sodium transition metal oxides, NaxMO2(M= Mn, Fe, Ni and Co etc.) (0.55<x<l), Olivine NaMPO4 (M = Fe, Mn, Ni and Co), spinel NaMn2O4 and NASICON- type (Na super-ionic conductor) Na3V2(PO4)3 have been investigated and are currently being used as cathode materials for SIBs. Layered type NaxM02are susceptible to structural instability during charge / discharge process due to oxygen evolution at high states of charge (SoC), which causes severe capacity degradation. In addition, poor thermal stability of the oxide materials inhibits their use at elevated temperature [N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, “Research Development on Sodium-Ion Batteries”, Chem. Rev. (2014) 114, 11636-11682],

[0006] Phosphate based materials are known to be structurally and thermally stable and can be used as potential electrode materials for SIBs. In particular, NASCION-type structure, which has open 3D ion transport channels to facilitate faster ion transportation, are widely studied as electrode materials. Na3V2(PC>4)3 of NASICON-type structure is an example of promising cathode material with a flat voltage plateau at 3.4 V vs. Na / Na+and high reversible specific capacity of 118 mAhg'1. However, the poor electronic conductivity associated with NASCION-type structure causes capacity degradation.

[0007] To address the poor electronic conductivity, various approaches such as electrically conductive coatings on particle surfaces and doping of elements to improve the conductivity, have been adopted using different chemical routes. Various synthesis routes adopted to prepare these materials were solid-state, sol-gel, electrospinning, freeze drying, hydrothermal and solvothermal methods. However, these synthesis routes involved formation of materials with secondary phases (such as in solid-state), or low yield and expensive (hydrothermal, electrospinning and freeze drying), or inhomogeneous gelation with prolonged reaction time. In addition, these above mentioned synthesis routes are not economical for the large scale synthesis of electrode materials with improved sodium-ion storage performance.

[0008] Therefore, there is a need for a cost-effective, simple and energy efficient method for synthesizing homogenous and uniform particles.OBJECTS OF THE PRESENT DISCLOSURE

[0009] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.

[0010] An object of the present invention is to provide a process for synthesizing in-situ carbon-coated sodium vanadium phosphate (Na^fPOA) with enhanced electrochemical properties for use as an electrode material in energy storage devices.

[0011] An object of the present invention is to provide an advanced electrode material for a wide range of energy storage devices, including lithium-ion and sodium-ion batteries, supercapacitors and hybrid energy storage devices.

[0012] Another object of the present invention is to provide an energy storage device having the in-situ carbon-coated sodium vanadium phosphate (Na^XAPO d as cathode material.SUMMARY

[0013] This summary is provided to introduce a selection of concepts in a simplified form that is further described below in the detailed description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0014] Aspects of the present disclosure relates to a low-cost, simple and energy-efficient additive assisted high energy milling process to synthesize homogenous and uniform particles in a very short reaction time. The disclosure also relates to application of sodium vanadium phosphate material in sodium ion batteries and sodium ion super capacitor for storage applications.

[0015] Accordingly, in an aspect, the present disclosure provides a process for synthesizing in-situ carbon coated sodium vanadium phosphate, comprising steps of: a) blending a sodium precursor, a vanadium precursor, a phosphorus precursor and a carbon precursor in a solvent to obtain a homogenous non-aqueous slurry; b) drying the homogeneous non-aqueous slurry at a predefined temperature and pressure to obtain a dried powder; c) carrying out high energy milling to the dried powder at 200-500 rpm to obtain a milled powder; and d) subjecting the milled powder to a two-stage heat treatment to obtain the in-situ carbon coated sodium vanadium phosphate (Na^XAPO A

[0016] In various embodiments, 1 to 5 wt.% of a process control agent / additive is utilized in the process at the step a) and step c) to avoid agglomeration of particles and as source ofin-situ carbon coating, respectively, wherein the process control agent is selected from a group consisting of stearic acid, hexane, methanol, ethanol and a combination thereof.

[0017] In certain embodiments, the sodium precursor is selected from a group consisting of sodium dihydrogen phosphate (NaH2PC>4), sodium acetate dihydrate (CH3COONa.2H2O), sodium hydroxide monohydrate (NaOH.H2O), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium phosphate (Na3PC>4), sodium phosphate dodecahydrate (Na3PC>4.12H2O), sodium oxalate (Na2C2C>4), and a combination thereof.

[0018] In certain embodiments, the vanadium precursor is selected from a group consisting of ammonium metavanadate (NH4VO3), vanadium oxide (V2O3). vanadic acid (HV03), vanadyl acetylacetonate [VO(C5H7O2)2], trihydroxy (oxo) vanadium (H3VC>4) and a combination thereof.

[0019] In various embodiments, the phosphorus precursor is selected from a group consisting of ammonium phosphate [(NH4)2HPO4], ammonium dihydrogen phosphate (NH4HJO4), phosphoric acid (H3PC>4), sodium dihydrogen phosphate (NaH2PC>4) and a combination thereof.

[0020] In various embodiments, the carbon precursor is selected from a group consisting of citric acid, ascorbic acid, oxalic acid, gluconic acid, and a combination thereof.

[0021] In certain embodiments, the solvent is selected from a group consisting of acetone, ethanol, isopropanol and toluene.

[0022] In various embodiments, the predefined temperature and pressure at step b) ranges from 80 to 120 °C and ambient pressure.

[0023] In certain embodiments, the high energy milling at step c) is carried out for a duration ranging from 1 to 5 hours.

[0024] In various embodiments, the two-stage heat treatment at step d) comprises carrying out calcination of the milled powder at 400 °C and 800 °C for 4 and 10 hours, respectively, under argon atmosphere; wherein during the calcination of the milled powder an in-situ carbon coating layer is formed on the sodium vanadium phosphate; wherein the in-situ carbon coating layer comprises a thickness ranging from 5 to 10 nm.

[0025] In certain embodiments, the synthesized in-situ carbon coated sodium vanadium phosphate comprises a particle size ranging from 300 to 500 nm.

[0026] In another aspect, the present disclosure provides an electrode material comprising the in-situ carbon coated sodium vanadium phosphate.

[0027] In various embodiments, the electrode material further comprises a conductive agent and a binder; wherein the conductive agent is selected from a group consisting ofgraphite, carbon blacks, conductive fibers, carbon nanotubes and graphene, in an amount ranging from 1 to 15 wt. %; wherein the binder is selected from a group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, and tetrafluoroethylene, in an amount ranging from 1 to 15 wt. %.

[0028] In yet another aspect, the present disclosure provides an energy storage device. The energy storage device includes a cathode, wherein the cathode comprises a current collector coated with the electrode material; an anode; a separator; and a non-aqueous electrolyte.

[0029] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.BRIEF DESCRIPTION OF DRAWINGS

[0030] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0031] FIG. 1 illustrates an exemplary method of synthesizing a carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure.

[0032] FIG. 2 illustrates an X-ray diffraction (XRD) pattern of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure.

[0033] FIG. 3 illustrates a Raman spectrum of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure.

[0034] FIG. 4 illustrates a photographic image of a scanning electron microscope (SEM) of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure.

[0035] FIG. 5 illustrates a cyclic voltammogram (CV) of carbon coated sodium vanadium phosphate, at a constant scan rate (100 pV / s) in the voltage window of 2.3-3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure.

[0036] FIG. 6 illustrates galvanostatic charge-discharge cycling plots for the carbon coated sodium vanadium phosphate at 0.1 C (1 C= 118 mA / g) in the voltage window of 2.3- 3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure.

[0037] FIG. 7 illustrates comparison of the capacity vs. cycle number plots for carbon coated sodium vanadium phosphate at 0.1 and 1.0 C-rates in the voltage window of 2.3-3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure.

[0038] FIG. 8 illustrates cyclic voltammetry plots at different scan rates for carbon coated sodium vanadium phosphate w.r.t. activated carbon (AC) in the voltage window of 0- 3.0 V and their corresponding specific capacitances, in accordance with an embodiment of the present disclosure.

[0039] FIG. 9 illustrates charge / discharge plots at different current densities for carbon coated sodium vanadium phosphate w.r.t. activated carbon (AC) in the voltage window of 0- 3.0 and their corresponding specific capacitances, in accordance with an embodiment of the present disclosure.DETAILED DESCRIPTION OF THE INVENTION

[0040] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention.

[0041] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0042] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0043] In some embodiments, numbers have been used for quantifying weight percentages, ratios, and so forth, to describe and claim certain embodiments of the invention and are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the writtendescription and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0044] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0045] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0046] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0047] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

[0048] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0049] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or otherelements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.

[0050] The description that follows, and the embodiments described therein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present invention. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0051] It should also be appreciated that the present invention can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.

[0052] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0053] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements a, b, and c, and a second embodiment comprises elements b and d, then the inventive subject matter is also considered to include other remaining combinations of a, b, c, or d, even if not explicitly disclosed.

[0054] Aspects of the present disclosure relates to a low-cost, simple and energy-efficient additive assisted high energy milling process to synthesize homogenous and uniform particles in a very short reaction time. The disclosure also relates to application of sodium vanadium phosphate material in sodium ion batteries and sodium ion super capacitor for storage applications.

[0055] Accordingly, in an aspect, the present disclosure provides a process for synthesizing in-situ carbon coated sodium vanadium phosphate.

[0056] FIG. 1 illustrates the process for synthesizing in-situ carbon coated sodium vanadium phosphate, including steps of: a) blending a sodium precursor, a vanadium precursor, a phosphorus precursor and a carbon precursor in a solvent to obtain a homogenous non-aqueous slurry; b) drying the homogeneous non-aqueous slurry at a predefined temperature and pressure to obtain a dried powder;c) carrying out high energy milling of the dried powder at 200-500 rpm to obtain a milled powder; and d) subjecting the milled powder to a two-stage heat treatment to obtain the in-situ carbon coated sodium vanadium phosphate (Na3V2(PO4)3).

[0057] In various embodiments, 1 to 5 wt.% of a process control agent / additive is utilized in the process at the step a) and step c) to avoid agglomeration of particles and as source of in-situ carbon coating, respectively. The process control agent is selected from a group consisting of stearic acid, hexane, methanol, ethanol and a combination thereof.

[0058] In certain embodiments, the sodium precursor is selected from a group consisting of sodium dihydrogen phosphate (NaH2PC>4), sodium acetate dihydrate (CH3COONa.2H2O), sodium hydroxide monohydrate (NaOH.H2O), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium phosphate (Na3PC>4), sodium phosphate dodecahydrate (Na3PC>4.12H2O), sodium oxalate (Na2C2C>4), and a combination thereof.

[0059] In certain embodiments, the vanadium precursor is selected from a group consisting of vanadium oxide (V2O5). ammonium metavanadate (NH4VO3), vanadic acid (HV03), vanadyl acetylacetonate [VO(CsH7O2)2], trihydroxy (oxo) vanadium (H3VC>4) and a combination thereof.

[0060] In various embodiments, the phosphorus precursor is selected from a group consisting of ammonium phosphate [(NH4)2HPO4], ammonium dihydrogen phosphate (NH4H2PO4), phosphoric acid (ITPO4), sodium dihydrogen phosphate (NaH2PC>4) and a combination thereof. The phosphorus precursors yield the frameworks of phosphates.

[0061] In various embodiments, the carbon precursor is selected from a group consisting of citric acid, ascorbic acid, oxalic acid, gluconic acid, and a combination thereof.

[0062] In certain embodiments, the solvent is selected from a group consisting of acetone, ethanol, isopropanol and toluene. Non-aqueous solvent is used for the preparation of the homogeneous non-aqueous slurry for the synthesis of carbon coated sodium vanadium phosphate.

[0063] In one embodiment, said carbon coated material includes at least one metal selected from a group consisting of vanadium, manganese, cobalt, iron, titanium and chromium.

[0064] In certain embodiments, the process further includes organic precursors for in-situ carbon coating which include but not limited to of glucose, sucrose, galactose, fructose, lactose, starch, mannose, ribose, aldohexose, ketohexose, and a combination thereof.

[0065] In one embodiment, while blending at step a) the sodium / phosphorous precursor, the vanadium precursor and the carbon precursor are present in a ratio of 1.5 : 1 : 1. The sodium precursor, the phosphorus precursor and the carbon precursor may be added by corresponding molar ratios considering the amount of the vanadium precursor.

[0066] In various embodiments, the blending at step a) is carried out in a closed vessel at a constant rotation speed using a rotary blender.

[0067] In various embodiments, the predefined temperature and pressure at step b) ranges from 80 to 120 °C and ambient pressure.

[0068] In certain embodiments, the high energy milling at step c) is carried out for a duration ranging from 1 to 5 hours.

[0069] In various embodiments, the two-stage heat treatment at step d) comprises carrying out calcination of the milled powder at 400 °C and 800 °C for 4 and 10 hours, respectively, under argon atmosphere. During the calcination of the milled powder, an in-situ carbon coating layer is formed on the sodium vanadium phosphate. The in-situ carbon coating layer comprises a thickness ranging from 5 to 10 nm. In one embodiment, the process includes heat- treatment of the milled powder in form of pellet at an optimized temperature and ambient pressure for the preparation of carbon coated sodium vanadium phosphate. In an embodiment, carbon coating or layer is formed from decomposition of organic precursors such as stearic acid, citric acid, oxalic acid, glucose, sucrose, ascorbic acid etc.

[0070] In one embodiment, uniform thickness of the in-situ carbon coating layer works as an excellent conductive framework for charge transfer and hence improves the sodium ion storage performance.

[0071] In certain embodiments, the synthesized in-situ carbon coated sodium vanadium phosphate comprises a particle size ranging from 300 to 500 nm. The in-situ carbon coated sodium vanadium phosphate particles have a NASICON structure. Process control agent may control the particle size and the particle size distribution of the carbon coated sodium vanadium phosphate, or regulating processing variables such as a milling time, reaction temperature and a reaction time, etc.

[0072] The process provides reduction in the reaction time due to the energy-efficient milling with uniform and controlled particle size of the carbon coated sodium vanadium phosphate. The process provided by the present invention address the prolonged reaction time, formation of phase pure materials and also the reduction in the cost, which are prevalent while adopting different synthesis routes, such as solid state reaction, hydrothermal, conventional sol-gel, electrospinning and freeze drying etc.

[0073] The process provided by the present invention is scalable to prepare pure phase material in bulk quantities (kilograms) within a short milling time for commercial applications.

[0074] In an embodiment, the process is used to synthesize non-limiting examples of electrode materials of alkali ion transition metal phosphates with general formula A3M2(PO4)3;where A= Li, Na, K and M= Fe, V, Ti, Mn, Co, Cr etc.

[0075] In an embodiment, the process is used to synthesize non-limiting examples of cation doped alkali ion transition metal phosphates with general formula A^I L-xC’xiPOAv 0<x<2, where C is at least one of the elements of Mg, Al, Ca, Sc, Cu, Zn, Ni etc.

[0076] In another embodiment, the process is used to synthesize non-limiting examples of anion doped alkali ion transition metal phosphates with general formula A3M2(PO4)3.xF3Xand A3M2.yCy(PO4)3-xF3x (x = 0-l).

[0077] In an embodiment, the process is used to synthesize non-limiting examples of electrode materials of multipolyanionic with general formula A4M3(PO4)2P2O7, where A= Li, Na; M= Fe, Mn, Co and alkali transition metal layered oxide materials with general formula AXM02, where A= Li, Na and M= Fe, Co, Ni, Cr, Mn, V, Ti etc.

[0078] In another aspect, the present disclosure provides an electrode material comprising the in-situ carbon coated sodium vanadium phosphate.

[0079] In various embodiments, the electrode material further comprises a conductive agent and a binder. The conductive agent is selected from a group consisting of graphite, carbon blacks, conductive fibers, carbon nanotubes and graphene, in an amount ranging from 1 to 15 wt. % based on total amount of the electrode material. The binder is selected from a group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, and tetrafluoroethylene, in an amount ranging from 1 to 15 wt. % based on total amount of the electrode material.

[0080] The conductive agent may include any material having high electronic conductivity. The graphite includes but not limited to natural graphite and synthetic graphite. The carbon blacks include as carbon black, acetylene black, and ketjen black. The conductive fibres includes a carbon fiber.

[0081] In one embodiment, the carbon coated sodium vanadium phosphate provided by the present invention can be used as positive cathode materials for sodium ion batteries. In one embodiment, the carbon coated sodium vanadium phosphate may be used as both positive and negative electrode materials in symmetrical sodium ion cells. In one embodiment, the carbon coated sodium vanadium phosphate may be used as electrodematerials for sodium ion capacitors. In one embodiment, the carbon coated sodium vanadium phosphate may be used as electrode materials for supercapacitor battery due to its dual characteristics of high specific capacity and high specific capacitance.

[0082] In yet another aspect, the present disclosure provides an energy storage device. The energy storage device includes a cathode, wherein the cathode comprises a current collector coated with the electrode material; an anode; a separator; and a non-aqueous electrolyte. The anode includes but not limited to Na metal. The non-aqueous electrolyte includes a sodium salt.

[0083] In one embodiment, the cathode for the energy storage device or a sodium ion battery obtained by coating the electrode material on the current collector. In one embodiment, the cathode for the sodium ion battery may be fabricated by, for example, dissolving the electrode material in a solvent, e.g. non-aqueous NMP (N-methyl-2- pyrrolidone) and aqueous (H2O) solvent to prepare a slurry, coating the slurry on the current collector, drying and calendaring.

[0084] The current collector of the energy storage device is made of any material having conductivity and may include, stainless steel, aluminum, nickel, carbon fdm or a surface treated material of aluminum or stainless steel with carbon etc.

[0085] Non-aqueous organic solvent for electrolyte may include aprotic organic solvents such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate and derivatives etc.

[0086] The sodium salts which are favourably soluble in the non-aqueous electrolyte may include NaC104, NaBF4, NaPF6, NaCF3SO3, NaCF3CO2, CF3SO3Na, and (CF3SO2)2NNa etc.

[0087] The energy storage device containing the prepared carbon coated sodium vanadium phosphate as electrode material specifically cathode shows good specific capacity of ~112 and 105 (±2) mAh / g and >90% cyclic stability.

[0088] While the foregoing description discloses various embodiments of the disclosure, other and further embodiments of the invention may be devised without departing from the basic scope of the disclosure. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.EXAMPLES

[0089] The present invention is further explained in the form of following examples. However, it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.Example 1: Method of synthesizing the carbon coated NasVzCPCDs

[0090] In-situ carbon coated sodium vanadium phosphate, Na^VzfPO-i)? was synthesized via additive assisted high energy milling. Stoichiometric amounts of each precursor were taken to prepare the Na^VzfPCLk. The precursors used were NaH2PC>4, NH4VO3 and citric acid (C’eHsO?). Stearic acid was used as the process control agent / additive for the synthesis of sodium vanadium phosphate during blending and high energy milling. The precursor stearic acid was dissolved in acetone, followed by addition of NaH2PC>4, NH4VO3 and citric acid (C’eH.sO?) precursors. The mixture was blended using zirconia ball with stainless steel container in a rotatory blender with material to ball ratio of 1:2. The prepared slurry was dried in a hot air oven at -100 °C, followed by high energy milling. The milling was carried out for 4 hrs at 300 rpm using SS container and balls. The material to ball ratio was maintained at 1: 10. The process control agent / additive of 5wt.% was added during the milling process. Finally, the milled powder was subjected to calcination at 400 °C and 800 °C for 4 and 10 hrs respectively, under argon atmosphere to get the single phase in-situ carbon coated Na3V2(PC>4)3. The material was prepared at large scale (1 kg / batch).Example 2: Characterization studies of the carbon coated sodium vanadium phosphate i) X-ray diffraction studies

[0091] FIG. 2 illustrates an X-ray diffraction (XRD) pattern of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure. The XRD pattern shows distinct peaks at specific 20 angles. The sharpness and intensity of these peaks indicate a crystalline structure. The highest intensity peak appears around 23°, followed by significant peaks near 28.5°, 32° and 35°. The XRD pattern confirms the crystalline nature of the carbon-coated sodium vanadium phosphate and indicates the presence of the intended Na3V2(PO4)3phase. ii) Raman Spectrum

[0092] FIG. 3 illustrates a Raman spectrum of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure. The Raman spectrumreveals the presence of carbon as confirmed from the D-band (1371 cm'1) and G-band (1608 cm'1). iii) Scanning electron microscope (SEM)

[0093] FIG. 4 illustrates a photographic image of a scanning electron microscope (SEM) of carbon coated sodium vanadium phosphate, in accordance with an embodiment of the present disclosure. It shows submicron sized particles in the range of 300-500 nm. iv) Cyclic voltammogram (CV)

[0094] FIG. 5 illustrates a cyclic voltammogram (CV) of carbon coated sodium vanadium phosphate, at a constant scan rate (0.1 mV / s) in the voltage window of 2.3-3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure. The graph shows clear peaks indicating the oxidation and reduction processes. These peaks represent the redox reactions occurring in the material. The oxidation peak (current going positive) occurs around 3.4 V, while the reduction peak (current going negative) is around 3.25 V. The CV curves for the 1st (black) and 2nd (red) cycles are relatively similar, which suggests good reversibility of the electrochemical reactions. The overlapping of the cycles indicates the material's stability and repeatability over multiple cycles. The maximum current observed during the oxidation process is around 0.6 mA, and during the reduction process, it goes down to approximately -0.6 mA. The CV indicates that carbon coated sodium vanadium phosphate exhibits good electrochemical activity and stability within the specified voltage window, making it a potentially suitable material for sodium-ion battery applications. v) Galvanostatic charge-discharge cycling

[0095] FIG. 6 illustrates galvanostatic charge-discharge cycling plots for the carbon coated sodium vanadium phosphate at 0.1 C (1 C= 118 mA / g) in the voltage window of 2.3- 3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure. The galvanostatic charge-discharge cycling plot demonstrates that C-Na3V2(PC>4)3 has a stable and relatively high specific capacity, making it a promising material for sodium-ion batteries. vi) Capacity ra. Cycle number

[0096] FIG. 7 illustrates comparison of the capacity vs. cycle number plots for carbon coated sodium vanadium phosphate at a) 0.1 C-rate and b) 1.0 C-rates in the voltage window of 2.3-3.9 V vs. Na / Na+, in accordance with an embodiment of the present disclosure. FIG. 7a shows that the specific capacity remains quite stable around 110 (±2) mAh / g over 35 cycles. FIG. 7b shows that the specific capacity is stable around 105(±2) mAh / g over 90 cycles. The carbon-coated sodium vanadium phosphate demonstrates excellent cycling stability at bothlow (0.1 C) and high (1.0 C) charge-discharge rates, with consistent specific capacities and minimal degradation over the tested cycles. vii) Comparative studies

[0097] FIG. 8 illustrates (a) cyclic voltammetry plots and (b) Corresponding specific capacitances at different scan rates for carbon coated sodium vanadium phosphate w.r.t. activated carbon (AC) in the voltage window of 0-3.0, in accordance with an embodiment of the present disclosure. FIG. 8a) shows that the CV curves for carbon coated sodium vanadium phosphate at various scan rates ranging from 1 mV / s to 100 mV / s. FIG. 8b) shows that at lower scan rates, the specific capacitance is higher, with values close to 100 F / g.

[0098] FIG. 9 illustrates (a) charge / discharge plots and (b) Corresponding specific capacitance at different current rates for carbon coated sodium vanadium phosphate w.r.t. activated carbon (AC) in the voltage window of 0-3.0, in accordance with an embodiment of the present disclosure. FIG. 9a) shows that the charge / discharge plots for carbon coated sodium vanadium phosphate at various current densities ranging from 0.1 A / g to 1 A / g. FIG. 9b) shows that at lower current density, the specific capacitance is higher, with values close to 67 F / g.ADVANTAGES OF THE PRESENT DISCLOSURE

[0099] The process provides a fast synthesis route for carbon coated sodium vanadium phosphate, significantly reducing the time required compared to traditional methods.

[0100] The in-situ carbon coating improves conductivity and stability of sodium vanadium phosphate, enhancing its electrochemical properties for better battery performance.

[0101] The process ensures uniform and controlled particle size distribution, which is crucial for consistent electrochemical performance and efficient charge / discharge cycles.

[0102] The process is scalable, enabling low-cost, large-scale production of carbon coated sodium vanadium phosphate, making it commercially viable for widespread application in energy storage devices.

[0103] The high energy milling and two-stage heat treatment are energy-efficient, leading to a more sustainable and environmentally friendly manufacturing process.

[0104] The carbon coated sodium vanadium phosphate can be used in various energy storage devices, including sodium-ion batteries, sodium-ion symmetric cells, and hybrid supercapacitors, due to its high specific energy and long-term stability.

Claims

We Claim:

1. A process for synthesizing in-situ carbon coated sodium vanadium phosphate, comprising steps of: a) blending a sodium precursor, a vanadium precursor, a phosphorus precursor and a carbon precursor in a solvent to obtain a homogenous non-aqueous slurry; b) drying the homogeneous non-aqueous slurry at a predefined temperature and pressure to obtain a dried powder; c) carrying out high energy milling of the dried powder at 200-500 rpm to obtain a milled powder; and d) subjecting the milled powder to a two-stage heat treatment to obtain the in-situ carbon coated sodium vanadium phosphate (Na3V2(PO4)3).

2. The process as claimed in claim 1, wherein 1 to 5 wt.% of a process control agent is utilized in the process at the step a) and step c) to avoid agglomeration of particles and as source of in-situ carbon coating, respectively, wherein the process control agent is selected from a group consisting of stearic acid, hexane, methanol, ethanol and a combination thereof.

3. The process as claimed in claim 1, wherein the sodium precursor is selected from a group consisting of sodium dihydrogen phosphate (NaH2PC>4), sodium acetate dihydrate (CH3COONa.2H2O), sodium hydroxide monohydrate (NaOH.H2O), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium phosphate (Na3PC>4), sodium phosphate dodecahydrate (Na3PC>4.12H2O), sodium oxalate (Na2C2C>4), and a combination thereof.

4. The process as claimed in claim 1, wherein the vanadium precursor is selected from a group consisting of vanadium oxide (V2O5). ammonium metavanadate (NH4VO3), vanadic acid (HV03), vanadyl acetylacetonate [VO(C5H7O2)2], trihydroxy (oxo) vanadium (H3VC>4) and a combination thereof.

5. The process as claimed in claim 1, wherein the phosphorus precursor is selected from a group consisting of ammonium phosphate [(NH4)2HPO4], ammonium dihydrogen phosphate (NH4H2PO4), phosphoric acid (H3PC>4), sodium dihydrogen phosphate (NaH2PC>4) and a combination thereof.

6. The process as claimed in claim 1, wherein the carbon precursor is selected from a group consisting of citric acid, ascorbic acid, oxalic acid, gluconic acid, and a combination thereof.

7. The process as claimed in claim 1, wherein the solvent is selected from a group consisting of acetone, ethanol, isopropanol and toluene.

8. The process as claimed in claim 1, wherein the predefined temperature and pressure at step b) ranges from 80 to 120 °C and ambient pressure.

9. The process as claimed in claim 1, wherein the high energy milling at step c) is carried out for a duration ranging from 1 to 5 hours.

10. The process as claimed in claim 1, wherein the two-stage heat treatment at step d) comprises carrying out calcination of the milled powder at 400 °C and 800 °C for 4 and 10 hours, respectively, under argon atmosphere; wherein during the calcination of the milled powder an in-situ carbon coating layer is formed on the sodium vanadium phosphate; wherein the in-situ carbon coating layer comprises a thickness ranging from 5 to 10 nm.

11. The process as claimed in claim 1, wherein the synthesized in-situ carbon coated sodium vanadium phosphate comprises a particle size ranging from 300 to 500 nm.

12. An electrode material comprising the in-situ carbon coated sodium vanadium phosphate as synthesized in claim 1.

13. The electrode material as claimed in claim 12, further comprises a conductive agent and a binder; wherein the conductive agent is selected from a group consisting of graphite, carbon blacks, conductive fibers, carbon nanotubes and graphene, in an amount ranging from 1 to 15 wt. %; wherein the binder is selected from a group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, and tetrafluoroethylene, in an amount ranging from 1 to 15 wt. 0 / / o.

14. An energy storage device, comprising: a cathode, wherein the cathode comprises a current collector coated with the electrode material of claim 12; an anode; a separator; and a non-aqueous electrolyte.

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