Host material for electrode, its process of preparation and applications thereof

Abstract

The present invention generally relates to the metal batteries. The present invention discloses sodium-plated host material comprising sodium-plated or bonded with metal modified electrospun carbon nanofiber, wherein the metal is selected from tin (Sn), silicon (Si), germanium (Ge), zinc (Zn), cobalt (Co) and lead (Pb). The present invention also discloses a process for preparation of sodium-plated host material and a full cell comprising the sodium- plated host material.

Description

[0001]HOST MATERIAL FOR ELECTRODE, ITS PROCESS OF PREPARATION AND APPLICATIONS THEREOF FIELD OF THE INVENTION The present invention generally relates to metal batteries. The present invention relates to a sodium-plated host material comprising sodium-plated or bonded with metal modified electrospun carbon nanofiber, and a process of preparation thereof. The present invention also relates to the half-cell and full cell containing the sodium-plated host material. BACKGROUND OF THE INVENTION As an alternative to lithium-ion batteries (LIBs), sodium (Na) is always the first candidate to be considered. Although commercial LIBs can deliver energy demands in almost every sector, the increased cost factor is due to the low abundance and geopolitical issues. Sodium is the most abundant metal on earth and can compensate for the significant economic problem associated with LIBs. Also, sodium shows the standard reduction potential of -2.71 V vs standard hydrogen electrode, and the theoretical specific capacity of 1166 mAh g-1, which encourages its applicability as an anode. Also, sodium anode with sulfur and oxygen cathode can put together to obtain high-energy batteries: Na-S and Na-O2, which have energy densities of 1274 Wh kg-1and 1605 Wh kg-1, respectively (Wang, L. et al., National Science Review 2022, 9 (3), nwab050). However, despite the several advantages of sodium, the hyper-reactivity of sodium causes a significant hindrance for the fabrication of batteries. Sodium can vigorously react with the electrolyte solvent to form the degradation products. Although it is essential to develop a passivation layer known as a solid electrolyte interface (SEI), the sodium salts formed in SEI get dissolved easily due to the low bulk modulus of sodium which is preferable (Bao, C. et al., Advanced Functional Materials 2020, 30 (52), 2004891). The unstable SEI leads to the problem of dendrites, which is the major factor for the failure of a cell due to a short circuit. The dendrites' main factor is the uneven deposition of sodium, which ultimately leads to its inhomogeneous growth on the substrate during the plating-stripping cycles (Li, Z. et al., Advanced Energy Materials 2022, 12 (4), 2100359). The research efforts to tackle the issues related to unstable sodium nucleation and growth includes developing an artificial SEI, by using additives in the electrolyte, modifying separators, and porous metal current collectors to compensate for the volume changes during cycling, and modifying host materials to plate sodium metal efficiently. There are some reports where said issues were addressed in publications such as i) Tao, L. et al., Advanced Functional Materials 2021, 31 (9), 2007556, which reports carbon nanofibers (CNFs) with different functional groups offering decent sodiophilicity. The interconnected ion network allows metallic sodium to be plated effectively on to the CNF support. The precursors used to prepare CNF also affect the functional groups, which interferes with the sodium deposition process. Mubarak, N. et al., Nano Energy 2021, 86, 106132 demonstrated a co-axial electrospinning setup to fabricate hollow, mesoporous carbon nanofibers. The CNF so formed are defect-rich and contain nitrogen functionalities to empower superior Coulombic efficiency (CE), diminished voltage hysteresis, and long cycle life while simultaneously accommodating large areal capacities of Na. However, these CNF-based materials suffer from limitations, i.e., the nucleation potential usually falls at higher voltages. Moreover, because of hyperactivity of sodium, the sodium metal anode (NMA) in sodium metal batteries suffers many challenges. One intrinsic problem with NMA is that dendrite growth occurs due to the spontaneous reaction of sodium with electrolyte forming fragile and unstable solid electrolyte interface (SEI) during plating and stripping, and volume variation leading to fluctuations in internal stress with poor interfacial stability and safety threat. As cycles increase, this sodium dendritic could traverse the separator to reach the cathode, causing internal short-circuiting. In view of said problems, there is a need of a sodiophilic host material with uniform deposition behavior for the effective NMA development. The sodiophilic material, which can decrease the nucleation overpotential, is expected to be a promising host material for sodium plating / stripping. Further, it is required to have a host material which can effectively control sodium metal deposition behavior with the motivation of engineering the metal host with uniformly distributed sodiophilic sites, presenting an efficient approach to develop dendrite- free and safe metal batteries. OBJECTIVES OF THE INVENTION An objective of the present invention is to provide a sodium-plated host material for sodium metal battery comprising sodium-plated or bonded with metal modified electrospun carbon nanofiber material. Another objective of the present invention is to provide a process for preparation of the sodium- plated host material. Another objective of the present invention is to provide an anode comprising the sodium-plated host material as coated layer on a current collector. The present invention also relates to a half-cell comprising the sodium-plated host material. The present invention also relates to a full cell comprising the sodium-plated host material. SUMMARY OF THE INVENTION The present invention relates to a sodium-plated host material for sodium metal battery comprising sodium-plated or bonded with metal modified electrospun carbon nanofiber, and a process of preparation thereof. The present invention also relates to a half-cell and full cell containing said sodium-plated host material. In an aspect, the present invention relates to a sodium-plated host material for sodium metal battery comprising sodium-plated or bonded with a metal modified electrospun carbon nanofiber, wherein the metal is selected from tin (Sn), silicon (Si), germanium (Ge), zinc (Zn), cobalt (Co) and lead (Pb). In another aspect, the present invention relates to a process for preparation of a sodium-plated host material by depositing sodium on a host material comprising metal modified electrospun carbon nanofibres at 0.5 to 2mA cm-2of current, and 0.5 to 2 mAh cm-2of capacity followed by de-crimping. a) electroplating sodium in a host material comprising metal modified electrospun carbon nanofiber using a Galvanostatic charge-discharge technique wherein the host material is kept under for long-term cycling of 300 h at 4 mA cm-2to 6 mA cm-2of current density with fixed capacity of 2 mAh cm-2, or b) depositing sodium on a host material at 0.5 to 2 mA cm-2of current, and 0.5 to 2 mAh cm-2of capacity followed by de-crimping. In another aspect, the present invention relates to a sodium metal anode comprising a sodium- plated host material. In another aspect, the present invention relates to a process for preparation of sodium metal anode by depositing the sodium-plated host material of the present invention on a current collector. In another aspect, the present invention relates to a full cell comprising a sodium metal anode of the present invention, a cathode, a separator and an electrolyte. Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments. BRIEF BESCRIPTION OF THE DRAWINGS: Figure 1 shows the sodium plating / stripping performance on the host material at 2 mAh cm-2for: (a) 4 mA cm-2, and (b) 6 mA cm-2. Figure 2 shows the pictorial representation of rate performance of Sn-CNF host material at different areal current densities. Figure 3 shows the Electrochemical impedance spectroscopy (EIS) plot for Na||Sn-CNF cell. Figure 4 shows the Full cell study of Na@Sn-CNF||NVPF for 30 cycles. Figure 5 shows the pictorial representation of nucleation barrier comparison of Sn-CNF at different current densities. Figure 6a & 6b shows the schematic representation of the coin cell battery comprising sodium- plated host material in a half cell and full cell. DETAILED DESCRIPTION OF THE INVENTION 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 disclosure as defined by the appended claims. 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. 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.” Also, use of "(s)" as part of a term, includes reference to the term singly or in plurality. It should also be appreciated that the present disclosure 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. The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 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. Unless otherwise indicated, the following definitions are set forth to illustrate and define the meaning and scope of the various terms used to describe the invention herein and the appended claims. These definitions should not be interpreted in the literal sense as they are not intended to be general definitions and are relevant only for this application. A “cell" is basic electrochemical unit that contains the electrodes, separator, and electrolyte. A “battery” is defined as a collection of cells or cell assemblies, with housing, electrical connections, and optionally additional elements for protection or control. A “dopant” refers to any metal additive that does not form intermetallic compounds with the primary metal in the amount used. “Cathode” refers to the electrode where reduction takes place during the discharge cycle. The cathode may be one continuous body, or it may include more than one thin film attached to a current collecting element. “Separator” refers to a barrier between the cathode and the anode to prevent them from coming into contact. If the electrolyte is solid, a separator is not necessary to avoid contact between the anode and cathode. “Electrolyte” refers to a solution containing a solvent and ions, which conducts ions but is an insulator to electrons. An electrolyte may be a liquid, solid, or gel. “Cycle” refers to the process of charging a rechargeable battery and discharging it as required into a load. The term is typically used to specify a battery's expected life, as the number of charge cycles affects life more than the mere passage of time. The term "or", as used herein, is generally employed in its sense including "and / or" unless the content clearly dictates otherwise. In an embodiment, the present invention provides a sodium-plated host material for sodium metal battery comprising sodium-plated or bonded with a metal modified electrospun carbon nanofiber, wherein the metal is selected from tin (Sn), silicon (Si), germanium (Ge), zinc (Zn), cobalt (Co) and lead (Pb). The metal modified electrospun carbon nanofiber comprises of metal in a range from 15 wt. % to 30 wt. % and the remaining is carbon nanofiber. In another specific embodiment, the metal is present at 15 wt. %, 17.5 wt. %, 20 wt. %, 22.5 wt. %, 25 wt. %, 27.5 wt. %, and 30 wt. %. The metal has diameter in a range from 15 nanometer (nm) to 20 nm. The carbon nanofiber has a diameter in a range from 130 nm to 150 nm. In some embodiments, the carbon nanofiber has a diameter in a range from 100 to 150 nm. The carbon nanofiber is in the form of nanotubes, nanosheets, and the like, having Brunauer-Emmett-Teller (BET) surface area in a range from 80 to 100 m2g-1. In an embodiment, the host material comprising the metal modified electrospun carbon nanofiber has a BET surface area in a range from 25 to 35 m2g-1. Further, the sodium-plated host material may comprise of dopants selected from lead (Pb), zinc (Zn), cobalt (Co) or combination thereof and is present in a range from 20 wt.% to 30 wt.%. The dopant may be employed to modify the carbon nanofiber and to enhance the sodiophilicity of the host material. The sodium-plated host material of the present invention is also referred to as Na-(M-CNF), wherein Na is sodium and M-CNF is the host material, comprising metal (M) embedded in the matrix of carbon nanofiber (CNF). The host material comprising the metal particles incorporate sodium (Na) in the CNF matrix thus providing Na-(M-CNF) structure. Typically, the nucleation potential of CNF usually falls at higher voltages, which can hinder efficient sodium deposition. Hence, the impregnation or embedding of metal in CNF reduces the nucleation potential, thereby promoting the nucleation and growth of sodium metal at lower voltages. The host material of the present invention promotes an instantaneous mode of sodium nucleation thereby improving the overall performance of sodium plating and stripping process. The metal (M) particles in the host material act as sodiophilic agents or sodiophilic sites, attracting sodium, reducing it to the metallic form, and thereby facilitating its uniform deposition / plating / bonding on the host material. Preferably, the metal is in the form of nanoparticles or nanoseeds embedded in the CNF (as metal-CNFs), which act as nucleation sites and effectively regulates the nucleation of sodium through alloying reactions between sodium and said metal. These alloying reactions contribute to a lower sodium nucleation potential and minimize energy barrier, associated with sodium deposition. Preferably, the host material is tin (Sn) modified electrospun carbon nanofiber (CNF). The tin particles in the form of nanoparticles or nanoseeds embedded in the CNF (as tin-CNF), helps regulate the nucleation of sodium onto the tin-based host material through alloying reactions between sodium and tin. Specifically, the Sn particles act as sodiophilic agents to attract sodium (Na), thereby depositing sodium on the host material (Sn-CNF). Tin metal is not replaced by Na. In another embodiment, the present invention provides a process for preparation of a sodium- plated host material by depositing sodium on a host material comprising metal modified electrospun carbon nanofibres at 0.5 to 2mA cm-2of current, and 0.5 to 2 mAh cm-2of capacity followed by de-crimping. The electrospun carbon nanofibers is obtained by known electrospinning techniques. The host material comprising metal modified electrospun carbon nanofibers is prepared by reacting sodiophilic metal precursors with CNF precursors. Since the sodiophilic metal precursors can be added in-situ with the CNF precursors during the synthesis of said CNF, it does not include an additional step of to the process. The resulting interconnected ion network of the metal-CNF host material facilitates efficient plating of metallic sodium on the said host material. Thus, the process of the present invention is simple and has reduced process steps. In step b), the process of decrimping comprises the removal of the sodium-plated host material from a coin cell. In a specific embodiment, the present invention provides a process for preparation of the sodium-plated host material comprising depositing sodium on the host material at 1mA cm-2of current, and 1 mAh cm-2of capacity followed by de-crimping. The present invention also provides a half-cell comprising a working electrode, a counter electrode, a separator, and an electrolyte. The working electrode comprises of metal modified electrospun carbon nanofiber (metal-CNF) as host material and is employed as cathode. The counter electrode is made of sodium metal and is employed as anode. The separator is selected from but not limited to glass microfiber separator, quartz microfiber, celgard membrane made of polypropylene, or a combination thereof. Preferably, the separator is glass microfiber separator. The electrolyte is selected from sodium hexafluorophosphate (NaPF6) in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 5% of fluoroethylene carbonate (FEC), NaPF6in diglyme, and sodium perchlorate (NaClO4) in solvent selected from carbonates (ester) and ether based organic solvents, sodium bis(trifluoromethanesulfonyl)imide in solvent selected from carbonate, ether, and ester based organic solvent. The carbonate (ester) solvent is selected from ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), fluoroethylene carbonate (FEC) or mixtures thereof. The ether-based organic solvent is selected from diethylene glycol dimethyl ether (diglyme), dimethoxyethane (DME), tetraethylene glycol dimethyl ether (TEGDME). Preferably, the electrolyte is NaPF6in diglyme. Preferably, the working electrode comprising the host material (metal-CNF) is deposited on a current collector in a form of a slurry and further comprises a conductive additive, a binder, and a solvent. The current collector is selected from copper foil, carbon-coated aluminium foil or combination thereof. The conductive additive is selected from but not limited to conducting carbon, fullerene (C65), ketjenblack carbon or combination thereof and is present in a range from 10 wt.% to 25 wt. %. The binder is selected from but not limited to polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), or a combination thereof and is present in a range from 5 wt. % to 15 wt. %. The solvent is selected from but not limited to N-methyl pyrrolidine (NMP), acetonitrile, water or a combination thereof. Preferably, the solvent is NMP. The working electrode comprising the host material of the present invention displays a low overpotential of 6.8 mV at 4 mA cm-2and 2 mAh cm-2. When the current density is increased to 6 mAcm-2with 2 mAhcm-2capacity, the working electrode displays an overpotential of 13.6 mV. This suggests the ability of the host material to facilitate uniform deposition of sodium at a low nucleation overpotential. In another embodiment, the present invention provides a sodium metal anode comprising the sodium-plated host material of the present invention. The sodium metal anode comprising sodium-plated host material of the present invention demonstrates a decreases voltage hysteresis due to alloying reactions between metal and sodium, thereby ensuring efficient and uniform plating / stripping of Na. In another embodiment, the present invention relates to a process for preparation of sodium metal anode comprising depositing the sodium-plated host material of the present invention on a current collector. The current collector is described above. The sodium-plated host material is deposited in an amount ranging from 55 wt. % to 75 wt. % as a thin layer on the current collector. Preferably, the sodium metal anode comprises sodium plated on tin modified electrospun carbon nanofiber (Sn-CNF) material with specific chemical and surface properties / characteristics as described above. In another embodiment, the present invention provides a full cell comprising: a) sodium metal anode as described above; b) a cathode; c) a separator; and d) an electrolyte. The cathode is selected from sodium vanadium fluorophosphate (Na3V2(PO4)2F3), sodium vanadium phosphate (Na3V2(PO4)3), sodium vanadium oxy fluoro phosphate (Na3V2(PO4)2 OF2). The separator and electrolyte are as described above. Preferably, the full cell comprises Na3V2(PO4)F3 (NVPF) as cathode and 1M NaPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 5% FEC as electrolyte with volume % ratio of EC and DEC of 1:1. Preferably, the full-cell and half-cell comprises of negative cell case and positive cell case and is in a form of a coin cell. Preferably, the coin cell is CR2032, having a height of 3.2 mm, and a diameter of 20 mm. The full cell comprising the sodium metal anode of the present invention demonstrates a stable rate capacity and high coulombic efficiency with reduced nucleation barrier, thus highlighting the effectiveness of the sodium metal anode comprising the sodium-plated host material of the present invention in maintaining uniform sodium deposition and mitigating issues such as dendrite formation and volume expansion with minimal side reactions during Na plating / stripping process. EXAMPLES The present invention is further explained in the form of the following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Example 1. Synthesis of Sn-CNF material (This synthesis protocol is disclosed in literature Badadhe, S. S. et al., Journal of Alloys and Compounds 2022, 907, 164318): The different chemicals used: tin chloride (II) dihydrate (Merck), dimethyl formamide (DMF) (Chem-lab), polyvinylpyrrolidone (PVP) (Sigma-aldrich), were of high purity and used without further purification. Herein, electrospinning technique was used for preparation of carbon nanofibers (CNFs), followed by annealing in controlled atmospheres to obtain tin-carbon nanofibers (CNF-Sn). For synthesis of carbon nanofibers, 2.4 g of PVP was dissolved in 20 ml of DMF and this solution was electrospun to obtain PVP fibers. The electrospinning was carried out at 20 kV with the flow rate of 1 ml / h. These PVP fibers were stabilized at 250 °C for 3 h air and then carbonized in presence of mixture of Ar and H2 at 700 °C to obtain the carbon nanofibers. Similar protocol was used to obtain Sn-CNF host material, wherein 1.7 g of tin chloride (II) dihydrate was stirred well in PVP solution (2.4 g of PVP in 20 ml of DMF) to form homogeneous solution, followed by electrospinning. Structural and morphological analysis: Typical X-ray diffraction patterns of as-prepared CNFs, and Sn-CNF samples are depicted herein. In case of CNFs, the XRD spectrum exhibits a broad peak centered at ~25⁰ along with an appearance of weak hump at ~45⁰, indicative of amorphous carbon. The XRD spectra of Sn- CNF samples depict a set of well-defined diffractions peaks and are indexed to (200), (101), (220), (311), and (301) planes of crystalline FCC phase of tin. As the intensities of characteristic signatures of amorphous carbon are weaker than that of tin metal. Thus, the XRD analysis reveals presence of amorphous carbon phase in CNFs, and crystalline FCC phase of tin metal in Sn-CNF sample. Example 2: Synthesis of sodium-plated host material (Sn-CNF) The Sn-CNF host material was deposited on a current collector. The sodium-plated Sn-CNF material was prepared by depositing sodium onto the Sn-CNF host material at 1mA cm-2and 1 mAh cm-2current and capacity respectively. Further cell was de-crimped, and the sodium- plated Sn-CNF (Na@Sn-CNF) was obtained. a) The sodium electro-plating / stripping on the host material was done using Galvanostatic charge-discharge technique. The metallic sodium disc was used as a reference and counter electrode, while Sn-CNF served as a working electrode. Wherein the host material is kept under for long-term cycling of 300h at 4 mA cm-2to 6 mA cm-2of current density with fixed capacity of 2 mAh cm-2to control the deposition / stripping time. Example 3 Preparation of half-cell assembly and full cell assembly Half cell: A coin-type test cell (CR2032) was utilized to evaluate the electrochemical performance of working electrode comprising Sn-CNF host material, kept towards the cathode side. The working electrode was prepared by using slurry consisting of 70 wt. % active material (Sn-CNF), 20 wt. % conductive carbon and 10 wt. % PVDF using NMP as a solvent. Sodium metal was used as a counter electrode on the anode side and a microporous glass fiber (Whatman, Cat. No.1825047, UK) was used as the separator. The electrolyte used for sodium cells was 1M (sodium hexafluorophosphate) NaPF6 in diglyme. Full cell: For full cell, 1 M NaPF6in a mixture (1:1, in vol %) of ethylene carbonate (EC) and diethyl carbonate (DEC) with 5% fluoroethylene carbonate (FEC) was used as electrolyte. The Na@Sn-CNF host material of Example 2 and was used as anode and paired with sodium vanadium fluorophosphate ((Na3V2(PO4)F3) (NVPF)) cathode, to fabricate a full cell. Metal anode cells were first discharged to 0.01 V at 25 mAg-1to form SEI before doing plating / stripping experiments. The cells were assembled in an argon-filled glove box (O2 level <0.1 ppm and H2O < 0.1 ppm). Example 4: Results of Half-cell and Full cell studies: The half-cell study was performed using constant current charge / discharge in battery tester to evaluate the performance of the host material by Na plating / stripping process. The sodium electro-plating / stripping on the host material was carried out using Galvanostatic charge- discharge technique. The metallic sodium disc was used as a reference and counter electrode, while Sn-CNF served as a working electrode. The host material was kept under for long-term cycling of 300h at 4 mA cm-2to 6 mA cm-2of current density with fixed capacity of 2 mAh cm-2to control the deposition / stripping time. Further the half-cell was tested for Electrochemical impedance spectroscopy (EIS) on a Biologic workstation. For full cell study, Na was initially deposited on Sn-CNF at 1 mA cm-2and 1 mAh cm-2. The Na@Sn-CNF was further used as an anode and paired with NVPF. The full cell study was performed based on the theoretical capacity of NVPF cycled at 0.1 C (1C= 128 mAh g-1) in constant current charge / discharge mode in battery tester. Galvanostatic charge-discharge (GCD) measurements were performed using MTI Corp. multi- channel battery test system. The results are provided in Table 1 below: Table 1 Cell Type Current Overpotential Capacity Cycles C.E. Density achieved Half Cell 4 mA cm-2at 6.8 mV - 240 cycles 98.6% (Na || Sn- fixed (200 h) CNF) capacity of 2 mAh cm-2Half Cell 6 mA cm-2at 13.4 mV - 300 cycles 96.93% (Na || Sn- fixed (200 h) CNF) capacity of 2 mAh cm-2Full Cell 12.8 mA g-1- 101 mAh g-130 cycles (Na@Sn- Acc. to the CNF|| theoretical NVPF) capacity of NVPF The sodium electroplating / stripping performance of the Sn-CNF material has been examined. Sodium Electrodeposition Testing for half-cell study (Table 1; Fig-1): Sodium plating / striping experiments were performed using Sn-CNF host material to establish the connection between surface properties and Na deposition in half-cell at different current densities and capacity. The nucleation overpotential, coulombic efficiency (C.E.) and cycle number are critical parameters to evaluate the sodiophilicity of the host material. Voltage vs. time curve of NaǀǀSn-CNF at 4 mA cm-2, 2 mAh cm-2and 6 mAcm-2with fixed capacity of 2 mAh cm-2is shown in Table 1 and Figure 1 (a) and (b). As observed from the said figure and Table 1, at 4 mA cm-2, with 2 mAh cm-2capacity (Figure 1(a)), NaǀǀSn-CNF displayed an overpotential of 6.8 mV with coulombic efficiency (C.E.) of 98.6% for over 240 cycles. When current density was increased to 6 mA cm-2with 2 mAh cm-2capacity (Figure 1(b)), NaǀǀSn- CNF displayed overpotential of 13.6 mV with C.E. of 96.93 % for over 300 cycles. This suggests that the high C.E and excellent stability of Sn-CNF host material is attributed to the uniform distribution of the sodiophilic seeds (Sn) in the CNF matrix which acts as nucleation sites, promoting uniform Na deposition while demonstrating lower overpotential. Non-uniform deposition of sodium on the host material may cause rupture of interface and instability. Hence, to determine the deposition of sodium, rate performance of Na plating / stripping behaviour was studied at different current densities of 1, 2, 4, and 6 mA cm-2with 2 mAh cm-2fixed capacity as depicted in Figure 2. NaǀǀSn-CNF exhibited stable C.E. of 100 % for cycles at different current rates which is attributed to the host material comprising Sn which act as nucleation sites and promote the uniform sodium deposition, by lowering the nucleation barrier for sodium deposition. The said Figure 2 also depicts the ability of the host material to tolerate sudden change in current which indicates its rapid reaction dynamics and adaptability of the material to varying operation conditions. Figure 3 depicts the EIS plot for Na||Sn-CNF half- cell which suggests the formation of solid electrolyte interface on the host material which is essential for stabilization of electrode surface and enabling subsequent reversible Na plating / stripping process. Electrochemical performance of sodium-plated host material of Example 2 in full cell: Subsequently, a proof-of-concept full cell comprising Na@Sn-CNF as anode and NVPF as cathode was fabricated and tested at 0.1C current rate (1C = 128 mAh g-1) as depicted in Figure 4 and Table 1. Figure 4 demonstrates the charging capacity, discharging capacity and C.E. of the Na@Sn-CNFǀǀNVPF full cell. The full cell comprising the sodium-plated host material of the present invention exhibited a stable capacity of 101 mAh g-1at 0.1 C rate over 30 cycles. This stability highlights the effectiveness of the Sn-CNF host material in maintaining uniform sodium deposition and mitigating issues such as dendrite formation and volume expansion. Further, the C.E. remains consistently high, close to 100% throughout the cycling process. This high efficiency indicates minimal side reactions and excellent reversibility of the sodium plating and stripping processes. The sodiophilic tin nanoparticles within the CNF matrix contributes to this performance by reducing the nucleation overpotential and ensuring stable cycling. Furthermore, to evaluate the barrier for the nuclei formation, the cell was discharged with two different current densities of 0.5 mA cm-2and 1 mA cm-2, at fixed capacity of 1 mAh cm-2as depicted in Figure 5. A very slight increase in the potential was observed in the negative region when the current density was increased, which is obvious as the applied current increases the overpotential also increases. It suggests the effective sodiophilicity of the Sn-CNF host material for the fast Na+deposition, reducing the nucleation barrier for sodium deposition under varying current densities. Figure 6(A) and 6(B) demonstrates the half-cell and full-cell configuration of the coin cell of Example 3. Figure 6B demonstrates the structure of a sodium metal battery comprising sodium metal anode of the present invention. Example 5 The electrochemical performance of the host material of the present invention for Na deposition was compared with conventionally known CNF materials and alloying materials, specifically used as a host material for the Na-metal anode in sodium metal batteries. A half-cell configuration was set-up according to Example 3 wherein the electrode comprising the different host materials as provide in Table 2 were taken as working electrode The results are demonstrated in Table 2 below: Table 2: Examples Materials Current (mA cm- Overpotential (mV) 2) / Capacity (mAh cm-2) / Coulombic Efficiency (%) Comparative Lignin-derived skeletal carbon 0.5 / 0.5 / 95 50 example A nanofiber (LCNF) (Advanced Functional Materials, 31(9), p.2007556) Comparative Hollow and mesoporous carbon 3 / 6 / 99.7 - example B nanofiber (HpCNF) (Nano Energy, 86, p.106132) Comparative Sn nanoparticles embedded within 2 / 5 / 99.3 - example C porous carbon network (Sn@C) (Journal of Materials Chemistry A 7, no. 41 (2019): 23747-23755) Comparative 2D freestanding frameworks of Sn 1 / 1 / 99.75 11.8 (10thcycle) example D nanoparticles embedded in (Nano Energy, 79, multilayer carbon sheets (Sn / C) 2 / 2 / 99.85 41.5-73.7 105457) Comparative A heterogeneous interfacial layer 5 / 5 / 99.5 42 example E having C-coated ZnO (Chemical microflowers and the Engineering encapsulated Sb nanocrystals Journal, 417, (Sb@ZMF / C) 128997) Comparative Polyacrylonitrile (PAN) fiber film 2 / 2 / 99.8 20 example F coated with a thin layer of tin as a (Advanced scaffold for Na deposition Energy (PAN / Sn) Materials, 10(44), 2002308) Present invention Sn-CNF host material 4 / 2 / 98.6 6.8 6 / 2 / 96.93 13.4 As observed from Table 2, Example 3 comprising the host material of the present invention, demonstrates a stable C.E. at higher current densities of 4 and 6 mA cm-2. Whereas, though Comparative Examples B, C, D and F, demonstrated a higher C.E., the same was achieved at a lower current density (0.5 to 3 mA cm-2). Thus, this suggest the superior rate capability of the host material of the present invention to demonstrate stable performance and uniform deposition of sodium even at high current densities, which is a key requirement for sodium metal batteries. Further, the present invention demonstrated a lower overpotential of 5-7 mV while Comparative Example A demonstrated an overpotential of 50 mV, Comparative example D demonstrated an overpotential of 41.5 mV to 73.7 mV, Comparative example E demonstrated an overpotential of 42 mV and Comparative examples F demonstrated an overpotential of 20 mV. Since, overpotential reflects the additional energy required for Na plating / stripping, a lower overpotential exhibited by present invention indicates fast Na-ion kinetics with reduced interfacial resistance and an efficient charge transfer. ADVANTAGES OF THE INVENTION • The present invention discloses a sodium-plated host material for sodium metal battery covering metal CNF based host material for uniform sodium plating. • It provides a sodiophilic host material with uniform deposition behavior for the sodium battery. • It provides sodium-plated host material, which can decrease the nucleation overpotential. • It provides a host material (e.g. Sn-CNF) which can effectively control sodium metal deposition behavior with the motivation of engineering the metal host with uniformly distributed sodiophilic sites. • It provides dendrite-free and safe metal (sodium) batteries.

Claims

We claim:

1. A sodium-plated host material for sodium metal battery, comprising a sodium plated or bonded with a metal modified electrospun carbon nanofiber, wherein the metal is selected from tin, silicon, germanium, zinc, cobalt and lead.

2. The sodium-plated host material as claimed in claim 1, wherein the metal modified electrospun carbon nanofiber comprising metal in a range from 15 wt. % to 30 wt. % and the remaining is carbon nanofiber.

3. The sodium-plated host material as claimed in claim 1, wherein the metal is tin (Sn).

4. The sodium-plated host material as claimed in claim 1, wherein the metal has a diameter in a range from 15 nm to 20 nm and the carbon nanofiber has a diameter in a range from 100 nm to 150 nm.

5. The sodium-plated host material as claimed in claim 1, wherein the surface area of the metal modified electrospun carbon nanofiber is in the range of 25 m²g-1to 35 m²g-1.

6. The sodium-plated host material as claimed in claim 1, wherein the sodium-plated host material further comprising dopants selected from lead (Pb), zinc (Zn) and cobalt (Co).

7. A process for preparation of the sodium plated host material, wherein the process comprising: a) depositing sodium on a host material comprising metal modified electrospun carbon nanofiber at 0.5 mA cm-2to 2 of current and 0.5 mAh cm-2to 2 mAh cm-2of capacity followed by de-crimping to obtain sodium plated host material.

8. A sodium metal anode, comprising the sodium plated host material as claimed in claim 1.

9. A process for preparation of the sodium metal anode, the process comprising depositing the sodium plated host material as claimed in claim 1 on a current collector.

10. A full cell comprising:a) sodium metal anode as claimed in claim 8; b) a cathode; c) a separator; and d) an electrolyte.

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