Description:FIELD OF THE INVENTION
The present invention relates to an electrode for sodium ion batteries. More specifically, the present invention relates an electrode comprising a molybdenum-based alluaudite frameworks material and a process for preparation thereof. The electrode comprising the molybdenum-based alluaudite frameworks material is used as an anode in sodium ion batteries.

BACKGROUND OF THE INVENTION
Sodium-ion batteries are widely being pursed for grid storage applications. For sodium-ion batteries, hard carbon (HC) is widely used as anode. Using simple Na (de)intercalation mechanism, HC can deliver robust electrochemical performance to be implemented in sodium-ion full cells. During anode operation of HC, the redox mechanism is little complex involving adsorption, intercalation, and simple Na metal deposition. Further, the synthesis of hard carbons heavily depends on the purity of the biomass precursors. In addition, the initial coulombic efficiency (ICE) of hard carbon is poor with huge irreversible capacity loss. In addition to hard carbons, several Ti-based oxides (e.g. Na2Ti3O7, Na2Ti6O13 etc.) are reported to work as intercalation based anodes for sodium-ion batteries. They offer good capacity albeit with low-rate kinetics. In contrast, phosphorous (black and red P), antimony (Sb), and bismuth (Bi) are also known to work as anodes for sodium-ion batteries involving (de)alloying mechanism. These anodes, based on alloying reaction, have high over potential and low-rate kinetics. This scenario offers room for development of new anode materials.

Alluaudites are the materials having a general formula A1A2M1(M2)2(XO4)3 where the A site is occupied by the alkali ions, M site denotes the transition metal site and X is the electronegative element like S, P, As, V, Mo, W and so on [Moore, P. B. Crystal chemistry of the alluaudite structure type: contribution to the paragenesis of pegmatite phosphate giant crystals. Am. Min.: J. of Earth Planet. Mater. 1971, 56 (11-12), 1955]. Alluaudite materials comprise of one alkali ion, one 3d transition metal centre (Fe, Co, Ni, Mn) and one polyanionic unit (like SO4, PO4, MoO4 etc.). Alluaudites consist of a 3D tunnel-like structure where the alkali ions (A= Na, Li, K) reside inside the tunnel which makes it easier for (de)intercalation of the alkali ions. Generally, in the case of sulfate and phosphate-based alluaudites, the transition metal centres (Fe, Co, Ni, Mn) participate in the redox reaction by changing their oxidation states. The first report on alluaudite class of cathode for sodium-ion batteries (SIBs) was presented in 2010 based on NaMnFe¬2(PO4)3. In line with this work, several PO4-based alluaudites were explored for SIBs. In 2014, Yamada group reported the first sulfate-based alluaudite, Na2Fe2(SO4)3, working as a 3.8 V cathode material. It benchmarks the highest Fe3+/Fe2+ redox active material for SIBs. It led to the development of sulfate containing alluaudite cathodes by optimizing the synthesis and electrochemical performance. In 2016, Ben Yahia et al. reported a vanadium-based alluaudite, NaMn2Fe(VO4)3, albeit with poor electrochemical activity with a specific capacity of 35 mAh/g. These reports were centered around the optimization of high-voltage cathode materials for SIBs. In 2017, Gao et al. reported the first molybdate-based alluaudite, Na2.67Mn1.67(MoO4)3, working as a 3.45 V cathode by using its Mn2+/Mn3+ redox couple. It also has a possible Mo6+/Mo5+ redox couple active ~2.4 V. However, such alluaudite materials have not been explored as anode for battery application. Moreover, there exists a requirement of non-toxic and sustainable materials that can work as low voltage anode in battery applications and offer high capacity.

OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide an electrode comprising a molybdenum-based alluaudite frameworks material.

Another objective of the present invention is to provide a process for the preparation of the electrode.

Another objective of the present invention is to provide a process for the preparation of the molybdenum-based alluaudite frameworks.

Another objective of the present invention is to utilize the electrode comprising molybdenum-based alluaudite frameworks as anode in sodium ion batteries.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended to determine the scope of the invention.

The present invention provides an electrode for sodium ion battery comprising a molybdenum-based alluaudite framework material having a general formula NaxMy(MoO4)3, wherein M is a transition metal; x has a value selected from 3.36, 3.6 and 4; and y has a value selected from 1, 1.2 and 1.32.

The present invention also provides a process for the preparation of an electrode as defined above comprises mixing the molybdenum-based alluaudite framework material with Super-P carbon black and a binder in a ratio of 70:20:10 to form a slurry; optionally adding water; and depositing the slurry on a substrate followed by drying to obtain the electrode.

The present invention also provides a sodium ion battery characterize in comprising an electrode comprising a molybdenum-based alluaudite framework material, as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 depicts Rietveld refinement of powder X-ray diffraction pattern (PXRD) collected at room temperature (?=1.5405 Å) depicting the presence of alluaudite (space group C2/c) Na3.6Ni1.2(MoO4)3 phase where the orange dots refer to the observed pattern, black line denotes the calculated pattern while the purple bar is the Bragg peak and green line denotes the difference between observed and calculated pattern. The structural illustration and the SEM image of the material are given in the inset.
Figure 2 depicts a. Galvanostatic (dis)charge voltage profile of Na3.6Ni1.2(MoO4)3 (NNMo) vs. Na at 10 mA/g current density within a voltage range of 0.01 V to 3.0 V, b. The dQ/dV plot (against Na) of NNMo is an irreversible phase transition during first cycle followed by reversible peaks upon subsequent cycling.
Figure 3 depicts rate kinetics study of NNMo alluaudite sample.
Figure 4 depicts study of cycling stability and coulombic efficiency of NNMo.
Figure 5 depicts potentiostatic intermittent titration technique (PITT) curves of NNMo alluaudite. a. during the first discharge up to 0.01 V, and b. during the second cycle of charge-discharge.
Figure 6 depicts a. Ex-situ XRD patterns of NNMo electrode at various state of (dis)charge, and b. Ex-situ XPS spectra of NNMo electrode taken different state of (dis)charge.
Figure 7 depicts Rietveld refinement of powder X-ray diffraction pattern (PXRD) collected at room temperature (?=1.5405 Å) depicting the presence of alluaudite (space group C2/c) Na3.36Co1.32(MoO4)3 phase where the black dots refer to the observed pattern, orange line denotes the calculated pattern while the green bar is the Bragg peak and brown line denotes the difference between observed and calculated pattern. The structural illustration and the SEM image of the material are shown in the inset.
Figure 8 depicts a. Galvanostatic (dis)charge voltage profile of Na3.36Co1.32(MoO4)3 (NCMo) vs Na at 10 mA/g current density within a voltage range of 0.01 V to 3.0 V, and b. The dQ/dV plot (against Na) of NCMo shows a similar behaviour like NNMo.
Figure 9 depicts the cycling stability and coulombic efficiency of NCMo upto 100 cycles.
Figure 10 depicts Study of cycling stability and coulombic efficiency upto 100 cycles.
Figure 11 depicts a. PXRD pattern in orange colour refers to the as-synthesized Na4Cu(MoO4)3 (NCuMo) which almost matches with the reference (green) pattern confirming the formation of alluaudite NCuM with the presence of little amount of impurities (shown by the *). b. The 3D tunnel-like alluaudite structure of NCuMo is shown where Cu is in an octahedral site and sharing the same position with Na1.
Figure 12 depicts a. Galvanostatic (dis)charge profile of alluaudite-structured Na4Cu(MoO4)3 (without any particle size optimization) at a current density of 10 mA/g. b. The corresponding dQ/dV plot of NCuMo showing Cu2+/Cu redox couple after the conversion.
Figure 13 depicts cycling stability and coulombic efficiency upto 100 cycles.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated process, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The term “some” as used herein is defined as “none, or one, or more than one, or all”. Accordingly, the terms “none”, “one”, “more than one”, “more than one, but not all” or “all” would all fall under the definition of “some”. The term “some embodiments” may refer to no embodiments or to one embodiment or to several embodiments or to all embodiments. Accordingly, the term “some embodiments” is defined as meaning “no embodiment, or one embodiment, or more than one embodiment, or all embodiments”.

More specifically, any terms used herein such as but not limited to “includes”, “comprises”, “has”, “consists” and grammatical variants thereof is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. The specification will be understood to also include embodiments which have the transitional phrase “consisting of” or “consisting essentially of” in place of the transitional phrase “comprising”. The transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim, except for impurities associated therewith. The transitional phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention.

Whether or not a certain feature or element was limited to being used only once, either way it may still be referred to as “one or more features” or “one or more elements” or “at least one feature” or “at least one element”. Furthermore, the use of the terms “one or more” or “at least one” feature or element do NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as “there NEEDS to be one or more” or “one or more element is REQUIRED”.

Use of the phrases and/or terms such as but not limited to “a first embodiment”, “a further embodiment”, “an alternate embodiment”, “one embodiment”, “an embodiment”, “multiple embodiments”, “some embodiments”, “other embodiments”, “further embodiment”, “furthermore embodiment”, “additional embodiment” or variants thereof do NOT necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.

As used herein, the term "about" is used to indicate a range or approximation that allows for slight variations or deviations from a specific value or parameter without departing from the scope of the present invention. When "about" is used in conjunction with numerical values, it signifies that the disclosed value or parameter may vary by ±10%, preferably ±5%, of the indicated values.

The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the invention.

Unlike other alluaudites, the Mo-based alluaudite can have two potential redox centres: one is the transition metal centre and the other one being Mo. Activating both these redox centres can impart high capacity. As molybdenum (Mo) can exhibit multiple oxidation states (+6, +5, +4, +2 and 0), the Mo-based alluaudites can in principle deliver multiple-electron redox reaction. Deviating from intercalation and alloying (as mentioned above), the molybdate involves conversion redox mechanism, which can offer high capacity with due material optimization.

Mo-based alluaudites work as a low voltage anode by exploiting Mo as redox centre. In the present invention, the anion Mo is in +6 oxidation state. The involvement of multiple electrons lead to higher specific capacity of the material. Upon discharging to low voltage, Mo show reversible anionic redox activity involving variable oxidation states to work as anode.

Further, the Mo-based compounds are non-toxic and sustainable for battery application.

In an aspect, the present invention provides an electrode for sodium ion battery comprising a Mo-based alluaudite framework material. The Mo-based alluaudite framework material works as electrochemically active anode materials for sodium-ion batteries. The underlying mechanism of these materials is probed by ex-situ XRD and ex-situ XPS technique. These Mo-based alluaudite framework based anodes involve a conversion reaction followed by a (de)insertion reaction. Initially, an irreversible conversion of the alluaudite materials occur to form different oxides and metals at the complete discharged state followed by further insertion of Na+ ions in the oxide phases.

The present invention provides an electrode for sodium ion battery comprising a molybdenum-based alluaudite framework material having a general formula NaxMy(MoO4)3, wherein M is a metal; x has a value selected from 3.36, 3.6 and 4; and y has a value selected from 1, 1.2 and 1.32.

In an embodiment of the present invention, the metal of the molybdenum-based alluaudite framework material is selected from a group comprising Ni, Co, Cu, Fe, Mn, and an iso-aliovalent dopant selected from Al and Mg.

The molybdenum-based alluaudite framework material is selected from Na3.6Ni1.2(MoO4)3, Na3.36Co1.32(MoO4)3, and Na4Cu(MoO4)3.

In an embodiment of the present invention, the electrode is an anode. The anode has a working potential ranging from 0.5 V to 0.6 V (vs. Na/Na+).

The present invention provides a process for the preparation of an electrode as defined above comprises mixing the molybdenum-based alluaudite framework material with Super-P carbon black and a binder in a ratio of 70:20:10 to form a slurry; optionally adding water; and depositing the slurry on a substrate followed by drying to obtain the electrode.

The binder is sodium carboxymethyl cellulose (NaCMC).

In an embodiment of the present invention the molybdenum-based alluaudite framework material is prepared by a process comprises:
combining a molybdate precursor, a precursor of a metal, and sodium nitrate with a fuel to form a mixture;
combusting the mixture to obtain a combustion product; and
annealing the combustion product to obtain the molybdenum-based alluaudite framework material.

The molybdate precursor, the precursor of the metal, and sodium nitrate to the fuel has a weight ratio of 1:1. The fuel is selected from glycine and citric acid.

In an embodiment of the present invention, the molybdate precursor, the precursor of the metal, and sodium nitrate are added in stoichiometric ratio according to the chemical formula of molybdenum-based alluaudite framework material.

The molybdate precursor is selected from ammonium molybdate tetrahydrate ((NH4)6Mo7O24.4H2O) and MoO3.

The precursor of the metal nickel (Ni) is selected from a group consisting of nickel nitrate tetrahydrate (Ni(NO3)2.4H2O), nickel acetate, nickel oxides, nickel sulfates, and nickel carbonates; the precursor of metal cobalt (Co) is selected from a group consisting of cobalt nitrate hexahydrate (Co(NO3)2.6H2O), cobalt acetate, cobalt oxides, cobalt sulfates, cobalt carbonates; and the precursor of metal copper (Cu) is selected from a group consisting of copper nitrate trihydrate (Cu(NO3)2.3H2O), copper acetate, copper oxides, copper sulfates, and copper carbonates.

The mixture of the molybdate precursor, the precursor of the metal, sodium nitrate and the fuel is combusted at a temperature in a range of 200 to 250 ? for 1 to 2 hours to obtain the combusted product.

The combusted product is annealed at a temperature in a range of 600 to 700 ? for 1 min to 24 hours.

The present invention also provides a sodium ion battery characterize in comprising an electrode comprising a molybdenum-based alluaudite framework material as defined above.

In an embodiment of the present invention, the sodium ion battery comprises:
a cathode;
an anode comprising the molybdenum-based alluaudite framework material; and
an electrolyte.

The cathode of the sodium ion battery of the present invention is selected from P3/P2 type layered oxides, (NaxMO2), NASICON Na3V2(PO4)3, and polyanionic Na4Fe3(PO4)2P2O7, and the electrolyte is selected from 1 M NaPF6 in EC:DEC electrolyte, 1 M NaClO4 in PC, 0.5 M NaPF6 in diglyme, 0.5 M NaPF6 in PC/EC, 1M NaTFSI in PC.

The anode of the sodium ion battery of the present invention has a working potential ranging from 0.5 V to 0.6 V (vs. Na/Na+).

In an embodiment of the present invention the sodium ion battery is a rechargeable sodium ion battery.

In an embodiment of the present invention, the anode comprising the Mo-based alluaudite framework material works for sodium-ion batteries within a voltage range of 0.01 V to 3.0 V.

In an embodiment of the present invention, the anode consisting of Na3.6Ni1.2(MoO4)3 and the anode consisting of Na3.36Co1.32(MoO4)3 exhibits a specific capacity of about 450 mAh/g and about 420 mAh/g (at a current density of 10 mA/g) respectively. The working potential of both the anodes consisting of Na3.6Ni1.2(MoO4)3 and Na3.36Co1.32(MoO4)3 is about 0.6 V (vs. Na/Na+).

In an embodiment of the present invention, the anode consisting of Na4Cu(MoO4)3 delivers a specific capacity of about 190 mAh/g with a working potential of about 0.5 V (vs. Na/Na+).

The electrode comprising Mo-based alluaudite materials showed good stability and rate kinetics upon cycling. The electrode comprising molybdenum-based alluaudite framework material Na3.6Ni1.2(MoO4)3, Na3.36Co1.32(MoO4)3 and Na4Cu(MoO4)3 showed excellent electrochemical performance as anodes in Na-ion batteries with a low working potential and moderate cyclability which can be further improved by optimizing particle size.

EXAMPLES:
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

The electrochemical properties were tested for the three different Mo-based alluaudite type of compounds: (a) Na3.6Ni1.2(MoO4)3, (b) Na3.36Co1.32(MoO4)3, and (c) Na4Cu(MoO4)3. The details are outlined below:

Example 1: Alluaudite structured Na3.6Ni1.2(MoO4)3 (NNMo) as a Na-ion battery anode.

Synthesis and Structure: Solution-combustion synthesis route was employed to synthesize NNMo material. Stoichiometric amounts of NaNO3, Ni(NO3)2.4H2O and molybdate precursors were used as the oxidizers along with glycine as the fuel maintaining the oxidizer-to-fuel ratio of 1:1 for combustion. The mixture was combusted at 200 ? for 1 hour to obtain a combustion product. The intermediate powder was annealed at 700 °C for 12 h to get the pure phase of NNMo. Phase analysis was performed using X-ray diffraction (at 25 °C) using a Panalytical X-ray diffractometer (with Cu Ka source). The refinement was performed using GSAS software with a reasonable fit (Figure 1). The material was found to crystallize in an alluaudite-type monoclinic crystal structure (space group: C2/c). Transition metal (M= Ni) makes M2O10 dimers form chains along the c-direction and those chains are connected by MoO4 tetrahedra to make a 2D sheet and in turn a 3D tunnel-like structure which is a typical structural characteristic of any alluaudite framework. The chemical reaction can be expressed as:

NaNO_3+Ni(NO_3 )_2.4H_2 O+?(NH_4)?_6 Mo_7 O_24.4H_2 O??Na?_3.6 ?Ni?_1.2 ?(MoO_4)?_3

Electrochemical Characterization: The active material (NNMo) was hand mixed with Super-P carbon black and aqueous binder (Sodium Carboxymethyl Cellulose, NaCMC) in a 70:20:10 ratio. Distilled water was used as a solvent to make a semi-thicker slurry. Afterward, this slurry was drop-casted on a precut 12 mm or 16 mm coupons made from battery grade Cu-foil. After drying in a vacuum oven overnight, the coatings were transferred into the Ar- filled glove box (MBraun LabStar GmbH, O2 and H2O levels below 0.5 ppm). The coatings were used against Na metal using commercially available 1M NaPF6 in EC:DEC (1:1 V/V, Kishida Chemicals) electrolyte to make CR2032 type coin cells. The galvanostatic tests were performed in a Neware BTS-4000A battery cycler within a voltage window of 0.01 V to 3.0 V, without any rest time in between charge and discharge. NNMo was found to act as a 0.6 V anode material for Na-ion batteries.

At 10 mA/g current densities, NNMo showed a specific discharge capacity of ~550 mAh/g on its first discharge (Figure 2a). On the subsequent cycle, it showed a specific charge and specific discharge capacity of ~450 mAh/g. After 20 cycles, it retained a specific charge capacity of ~250 mAh/g and a specific discharge capacity of ~300 mAh/g. The corresponding dQ/dV plot is shown in Figure 2b which shows a sharp irreversible peak at ~0.87 V while the other peaks are reversible in subsequent cycles. From the charge-discharge profile and their derivative plots of the material, it is evident that there is some kind of irreversible phase transition happening at the first discharge which corresponds to the peak present at 0.87 V for NNMo.

The rate kinetics of the material shown in Figure 3, showed moderate rate kinetics mostly due to the large volume change during the cycling.

After 100 cycles of charge-discharge, NNMo showed a specific charge capacity of ~150 mAh/g and a specific discharge capacity of ~100 mAh/g (Figure 4). The material showed an excellent Coulombic efficiency of nearly 100% for long term cycling. It suffers from poor capacity retention after 100 cycles. The cycling stability can be improved by optimizing the particle size or using different electrolytes.

Finally, to probe the mechanism, potentiometric titration technique (PITT), ex-situ XRD and ex-situ XPS technique were used. From the potentiometric titration technique, it was observed that on the first discharge (Figure 5a), there is a presence of non-Cottrellian behaviour in the current within a voltage range of 1.0 V to 0.6 V. This gives a hint towards the phase change happening within this voltage cut-off might be possible due to an irreversible conversion. In the second cycle (Figure 5b), this non-Cottrellian region is missing. While another non-Cottrellian region within 0.5 to 0.3 V is reproducible in subsequent cycles. The PITT hinted of an irreversible phase transition (conversion) followed by a reversible insertion mechanism.

Ex-situ XRD and XPS were performed at different states of (dis)charge of NNMo. For all the ex-situ analyses, swagelok-type cells were made and post cycling, the Swageloks were disassembled inside glovebox, the electrodes were washed using anhydrous diethyl carbonate (DEC). As evident from the ex-situ XRD (Figure 6a), the alluaudite phase stays intact during discharging until 1.6 V. While discharging down to 0.7 V, we can see a prominent change. The alluaudite peaks vanished along with the appearance of new XRD peaks at ~12.5°, ~16°, ~22.5°, ~28° and so on corresponding to some mixture of different oxides formed after the conversion. The phase at the end of first discharge at 0.01 V could not be identified from the XRD. But it was observed that phases formed at 0.7 V at its first discharge can reversibly intercalate the Na+ ions in and out of the structures. After a full charge upto 3.0 V, subsequent discharge upto 0.65 V (2D-0.65 V) leads to similar XRD profile (1D- 0.7 V). From the ex-situ XPS study of Mo, there is no shift in the Mo peaks when it was discharged upto 1.6 V retaining its pristine 6+ oxidation state. While deep discharging down to 0.01 V, the presence of Mo4+ as well as some amount of Mo metal (Figure 6b) can be identified. A similar mechanism is expected for its Co- analogue, NCMo since the electrochemical charge-discharge profiles is absolutely similar to the NNMo phase. From both the ex-situ XRD and XPS, the underlying redox mechanism can be expressed as following:

Within 3.0 V to 1.0 V, ?Na?_2 M_2 ?(MoO_4)?_3+?Na?^++e^-??Na?_(2+x) M_2 ?(MoO_4)?_3 (M= Ni)

Within 1.0 V to 0.5 V, ?Na?_(2+x) M_2 ?(MoO_4)?_3+?Na?^++e^-??Na?_2 O+MO+MoO_x

Within 0.5 V to 0.01 V, MO+MoO_x+?Na?^++e^-= ?Na?_2 O+ M+Mo (reversible)

Both XRD and XPS hint towards a conversion reaction followed by (de)insertion mechanism.

Example 2: Alluaudite type Na3.36Co1.32(MoO4)3 (NCMo) as a Na-ion Battery anode

Synthesis and structure: Solution-combustion synthesis route was employed to synthesize NCMo material. Stoichiometric amount of NaNO¬3, Co(NO3)2.6H2O and molybdate precursors were used as the oxidizers along with glycine as the fuel maintaining the oxidizer-to-fuel ratio of 1:1 for combustion. The mixture was combusted at 200 ? for 1 hour to obtain a combustion product. The final calcination for NCMo was conducted at 600 °C for 1 minute. Phase analysis was performed by taking a PXRD pattern using a Panalytical X-ray diffractometer (with Cu Ka source). The refinement was performed using GSAS software with a reasonable fit (Figure 7). It was found to crystallize in same alluaudite structure like its Ni-analogue. The chemical reaction is given below:

NaNO_3+Co(NO_3 )_2.6H_2 O+?(NH_4)?_6 Mo_7 O_24.4H_2 O??Na?_3.36 ?Co?_1.32 ?(MoO_4)?_3

Electrochemical Characterization: The active material (NCMo) was hand mixed with Super-P carbon black and aqueous binder (Sodium Carboxymethyl Cellulose, NaCMC) in a 70:20:10 ratio. Distilled water was used as a solvent to make a semi-thicker slurry. After that, the slurry was drop-casted on a precut 12 mm or 16 mm coupons made from battery grade Cu-foil. After drying in vacuum oven for overnight, the coatings were transferred into the Ar- filled glove box (MBraun LabStar GmbH, O2 and H2O levels below 0.5 ppm). The coatings were used against Na metal using commercially available 1M NaPF6 in EC:DEC (1:1 V/V, Kishida Chemicals) electrolyte to make CR2032 type coin cells. The tests were performed in a Neware BTS-4000A battery cycler within a voltage window of 0.01 V to 3.0 V, without a rest time in between charge and discharge. NCMo was found to act as 0.6 V anode material in Na-ion batteries.

Like NNMo, NCMo also showed a specific discharge capacity of ~550 mAh/g on its first discharge (Figure 8a) at 10 mA/g current density. On the subsequent cycle, it showed a specific (dis)charge capacity of ~420 mAh/g (Figure 8a). After 20 cycles, it retained a specific charge capacity of ~270 mAh/g and a specific discharge capacity of ~300 mAh/g. The corresponding dQ/dV profile (Figure 8b) looks similar to that of NNMo hinting towards a similar electrochemical redox mechanism.

The rate kinetics of the material in Figure 9 showed a moderate rate kinetics mostly due to the large volume change during the cycling.

After 70 cycles, NCMo showed a specific charge capacity of ~220 mAh/g and a specific discharge capacity of ~160 mAh/g (Figure 10). The material showed excellent Coulombic efficiency nearly upto 100% for long term cycling. NCMo suffer from a poor capacity retention after 70 cycles while it can be improved either by optimizing the particle size or using different electrolyte.

Example 3: Alluaudite type Na4Cu(MoO4)3 (NCuMo) as a Na-ion battery anode

Synthesis and structure: Solution-combustion route was employed to synthesize the target material. Stoichiometric amount of NaNO¬3, Cu(NO3)2.3H2O, (NH4)6Mo7O24.4H2O were used as the oxidizers and citric acid was used as the fuel to form a mixture. The mixture was combusted at 200 ? for 1 hour to obtain a combustion product. The combustion product was annealed at 630 °C for 24 hours to get the alluaudite NCuMo product. Nominal amount of unidentified impurities were always found with the dominant alluaudite NCuMo phase (Figure 11a). The structure is absolutely similar to the Ni and Co analogues (Figure 11b). The chemical equation for the reaction is as follows:

NaNO_3+Cu(NO_3 )_2.3H_2 O+?(NH_4)?_6 Mo_7 O_24.4H_2 O??Na?_4 Cu?(MoO_4)?_3

Electrochemical Characterization: The active material (NCuMo), conductive carbon (Super P) and CMC binder were mixed together (70:20:10 ratio) in presence of distilled water to make a slurry which was further drop-casted on Cu-foil further cut into 12 mm or 16mm coupons. CR2032 type coin cells were assembled inside an Ar-filled glove box against Na metal with 1 M NaPF6 in EC:DEC electrolyte. The cells were tested in between the voltage range of 0.01 V to 3.0 V. At 10 mA/g current density, the material showed a specific charge capacity of ~180 mAh/g with a working potential ~0.5 V (Figure 12a). The corresponding dQ/dV plot shows a similar profile like NCMo and NNMo further giving a hint towards similar redox mechanism as mentioned in Case 1 and Case 2 (Figure 12b).

Without any particle size optimization, the material showed an excellent stability with a capacity retention upto ~100 mAh/g even after 100 cycles with an excellent coulombic efficiency (~99%) (Figure 13). Overall, these Mo-based alluaudite materials can act as potential anodes for secondary sodium-ion batteries.

Advantage of the present invention over prior art.
The present invention provides an electrode comprising molybdenum-based alluaudite framework material for sodium batteries. These anode comprising molybdenum-based alluaudite materials can be to make safe Na-ion batteries with a low working potential of around 0.5- 0.6 V.
, Claims:1. An electrode for sodium ion battery comprising a molybdenum-based alluaudite framework material having a general formula NaxMy(MoO4)3, wherein M is a transition metal; x has a value selected from 3.36, 3.6 and 4; and y has a value selected from 1, 1.2 and 1.32.

2. The electrode as claimed in claim 1, wherein the molybdenum-based alluaudite framework material comprises a metal selected from Ni, Co, Cu Fe, Mn, and an iso-aliovalent dopant selected from Al and Mg.

3. The electrode as claimed in claims 1 and 2, wherein the molybdenum-based alluaudite framework material is selected from Na3.6Ni1.2(MoO4)3, Na3.36Co1.32(MoO4)3, and Na4Cu(MoO4)3.

4. The electrode as claimed in claim 1, wherein the electrode is an anode, wherein the anode has a working potential ranging from 0.5 V to 0.6 V (vs. Na/Na+).

5. A process for the preparation of an electrode as defined in claims 1- 4, comprises mixing the molybdenum-based alluaudite framework material with Super-P carbon black and a binder in a ratio of 70:20:10 to form a slurry; optionally adding water; and depositing the slurry on a substrate followed by drying to obtain the electrode.

6. The process as claimed in claim 5, wherein the binder is sodium carboxymethyl cellulose (NaCMC).

7. The process as claimed in claim 5, wherein the molybdenum-based alluaudite framework material is prepared by a process comprises:
i. combining a molybdate precursor, a precursor of a metal, and sodium nitrate with a fuel to form a mixture;
ii. combusting the mixture to obtain a combustion product; and
iii. annealing the combustion product to obtain the molybdenum-based alluaudite framework material.

8. The process as claimed in claim 7, wherein the molybdate precursor, the precursor of the metal, and sodium nitrate to the fuel has a weight ratio of 1:1; wherein the fuel is selected from glycine and citric acid; wherein the molybdate precursor is selected from ammonium molybdate tetrahydrate ((NH4)6Mo7O24.4H2O) and MoO3.

9. The process as claimed in claim 7, wherein the precursor of the metal nickel (Ni) is selected form a group comprises nickel nitrate tetrahydrate (Ni(NO3)2.4H2O), nickel acetate, nickel oxides, nickel sulfates, and nickel carbonates; the precursor of the metal cobalt (Co) is selected form a group comprises cobalt nitrate hexahydrate (Co(NO3)2.6H2O), cobalt acetate, cobalt oxides, cobalt sulfates, cobalt carbonates; and the precursor of the metal copper (Cu) is selected form a group comprises copper nitrate trihydrate (Cu(NO3)2.3H2O), copper acetate, copper oxides, copper sulfates, and copper carbonates.

10. The process as claimed in claim 7, wherein the mixture is combusted at a temperature in a range of 200 to 250 ? for 1 to 2 hours.

11. The process as claimed in claim 7, wherein the annealing is carried out at a temperature in a range of 600 to 700 ? for 1 min to 24 hours.

12. A sodium ion battery characterize in comprising an electrode comprising a molybdenum-based alluaudite framework material as claimed in claims 1- 4.