DESC:FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & The Patent Rules, 2003 COMPLETE SPECIFICATION (See sections 10 & rule 13) 1. TITLE OF THE INVENTION A STABLE POSITIVE ELECTRODE COMPOSITE MATERIAL FOR SODIUM ION BATTERIES AND ITS PREPARATION METHOD THEREOF 2. APPLICANT (S) NAME NATIONALITY ADDRESS INDIGENOUS ENERGY STORAGE TECHNOLOGIES PVT. LTD. IN I-10, 2nd Floor, Tides Business Incubator, IIT Roorkee, Roorkee-247667, Uttarakhand, India. 3. PREAMBLE TO THE DESCRIPTION COMPLETE SPECIFICATION The following specification particularly describes the invention and the manner in which it is to be performed. FIELD OF INVENTION: [001] The present invention relates to the field of sodium ion batteries (SIBs). The present invention in particular relates to the synthesis of novel nitrogen-doped carbon coated specific metal ions co-doped pure phase Na3V2(PO4)2F3 (NVPF) which is a cathode material for sodium-ion batteries that exhibits significantly improved electrochemical performance. DESCRIPTION OF THE RELATED ART: [002] A secondary battery, which has a low production cost and is based on sodium widely distributed on the earth, may be considered the most suitable energy storage source for the next-generation energy storage system. In addition, sodium is an element belonging to the same group of the periodic table as lithium, and the sodium-based secondary battery has an advantage in that it may be produced using an existing production base which has been used for a lithium secondary battery, because it is similar to the lithium secondary battery in terms of the electrochemical behavior and production method thereof. [003] In recent years, research on sodium-based secondary batteries using sodium instead of lithium has been re-examined. The reserves of sodium resources are abundant, and if it is possible to produce a secondary battery using sodium instead of lithium, the secondary battery may be produced at low costs. In addition, expensive nickel and cobalt may not be used as positive electrode materials for sodium ion batteries, unlike lithium ion batteries that essentially use nickel and cobalt as positive electrode materials. [004] However, sodium secondary batteries have physical properties different from existing lithium secondary batteries, and thus show low properties in terms of output power and energy storage density. An energy storage system is a system that stores power, generated by solar cells or wind power generation, at a time when the demand therefor is small, and then supplies energy at a time when energy demand rises sharply. For efficient driving, it is important to construct a large-capacity storage system at a relatively low cost rather than a storage system having high energy and power densities. [005] Therefore, studies have been actively conducted on positive electrode active materials which are important materials for commercialization of such sodium ion batteries. In particular, among various positive electrode materials, polyanion-based Na3V2(PO4)2F3 (NVPF), Na3V2O2(PO4)F (NVOPF) and Na3V2(PO4)3 (NVP), which have high energy density and power density and also have excellent cycle stability, have been intensively studied. [006] Reference may be made to the following: [007] IN Publication No. 202111047336 relates to a method of in-situ synthesis of nitrogen doped carbon coated sodium vanadium fluorophosphates (N-doped carbon coated NVPF) material. The material provided is 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. [008] Publication No. US2021242451 relates to a sodium-ion storage material including a doped compound, and an electrode material for a sodium-ion battery, an electrode for a sodium-ion battery, and a sodium-ion battery, which include the sodium-ion storage material. Specifically, the sodium-ion storage material may include a compound consisting of an Na3V2-xMx (PO4)2F3/Na3V2-yMy (PO4)3 composite (M=Fe, Mn, Cr, Cu, Zn or Ti, 0, Cl<->, SO4<2->, BO3<3->, PO4<3-> and SiO4<4->. According to the anion-doped sodium ion battery oxide cathode material, anion doping is introduced in a sodium ion battery oxide cathode material, the stability of the structure of the material can be maintained, the electrochemical performance of the material is improved, and the material has very good application prospects. [019] Publication No. CN106058202 relates to a carbon-coated metal ion-doped sodium vanadium phosphate composite cathode material prepared by a freeze drying method, as well as a preparation method and application thereof. A preparation process comprises the following steps: completely dissolving a vanadium source, a sodium source, phosphate, a citric acid and a metal ion source in de-ionized water, and freezing a mixed solution into ice after uniform stirring; performing freeze drying on a frozen ice block in a freeze dryer to obtain a drying product; performing heat treatment on the drying product in the air to obtain sodium vanadium phosphate precursor powder; uniformly grinding the powder, and performing heat treatment cooling in a protective gas atmosphere to obtain the material. The material is applied to a sodium ion battery cathode material, the electrochemical performance of the material is improved by the porous structure of the material, and the electrical conductivity and the structural stability of the sodium vanadium phosphate material can be effectively improved by ion doping. Compared with undoped sodium vanadium phosphate and a material for which the freeze drying method is not adopted, the prepared material has higher charge and discharge specific capacity and higher cycling stability. [020] Publication No. KR20220008612 relates to a positive electrode active material for a sodium ion battery, which comprises: a positive electrode material containing Na(1 - x)TMO_2 (0.66 = x = 1, and TM is at least one transition metal forming charge neutrality selected from Ni, Mn, Fe, Co, and Cr); and a Mg_(1 - x)Ni_xO coating layer coated on the surface of the positive electrode material, wherein the surface of the positive electrode material relates is doped with Mg^2+. Accordingly, surface deterioration and structural instability of a positive electrode material containing Na(1-x)TMO_2 (0.66 = x = 1, and TM is at least one transition metal forming charge neutrality selected from Ni, Mn, Fe, Co, and Cr) can be reduced and lifespan characteristics and rate characteristics of a sodium ion battery introducing the same can be increased. [021] Publication No. AU2021103590 relates to a sodium manganese fluorosilicate-based cathode material for a sodium ion battery, having a molecular formula of Na3AXMnix)SiO4F, where A is a doped metal ion selected from the group consisting of Mg2, Ca2, Sr2+ , Fe2, C02+, Ni2+, Cu2+ or Zn, and x=0 0.05. The sodium manganese fluorosilicate-based cathode material has high reversible capacity, and relatively desirable cycle performance. The present disclosure further discloses a preparation method of the sodium manganese fluorosilicate-based cathode material for a sodium ion battery. 140 -- * 0 2 4 6 8 10 Number of cycles [022] Publication No. WO2020235909 relates to a cathode active material for a sodium secondary battery, wherein the cathode active material has a 03 crystalline structure, and calcium is doped onto a sodium layer of a sodium transition metal oxide so as to stabilize a structure during a charging/discharging process, and thus battery performance is improved and driving is possible even under a high voltage exceeding 4.0 V. [023] Publication No. US2016013470 relates to a sodium transition metal cathode material for a rechargeable sodium battery having a P2 layered bronze crystal structure, comprising at least 55 mol % manganese, wherein the manganese valence state is at least 3.75. The material undergoes a structural transformation to a secondary cathode material by extraction of sodium during the 1st charge of a rechargeable sodium battery comprising the sodium cathode material. The material has either a composition NaxMO2 where M=Mn1-y-zLiyAz where z<0.2 and y<0.33 and 0.66