Description:FIELD OF THE INVENTION
The present invention relates to an Fe-based mixed polyanionic material. More specifically, the present invention relates to a process for the preparation of the Fe-based mixed polyanionic material through spray drying technique. The Fe-based mixed polyanionic material has sodium (Na) super ionic conductor (NASICON)-type structure and is utilized as cathode in lithium-ion batteries.

BACKGROUND OF THE INVENTION
Energy stands as an indispensable necessity for humanity, and the pressing challenges posed by the limitations of fossil fuels and the escalating concerns of global warming, exacerbated by population growth, have intensified the quest for renewable energy sources and efficient energy storage methods. One pivotal player in this quest is electrical energy storage (EES) devices, which play a critical role in storing energy from renewable sources as chemical energy for future use. This environmentally friendly approach, reliant on chemical reactions involving the transfer of electrons, holds great promise. Among EES devices, rechargeable batteries, in particular, stand out as highly appealing, converting chemical energy into electrical energy through a reversible process. At the forefront of battery technology, lithium-ion batteries (LIBs) dominate this field due to their remarkable energy density and cycling stability. The widespread adoption of portable consumer electronic devices has been made possible by the development of LIB technology. Currently, this technology is propelling the electric vehicle revolution and grid storage sector. The characteristics of cathodes used in LIBs, encompassing high energy density, cost-effectiveness, environmental friendliness, and sustainability, are pivotal factors for their extensive deployment.

Two-dimensional layered oxide materials have long been fundamental cathode components in LIBs [Wu, E. J.; Tepesch, P. D.; Ceder, G., Size and charge effects on the structural stability of LiMO2 (M = transition metal) compounds. Philos. Mag. B. 1998, 77 (4), 1039-1047]. These cathodes meet the requirement for high energy density and higher average working potential. Still, there are drawbacks related to elevated cost and scarcity of resources with respect to oxides containing Co and Ni transition metals. In this context, mixed polyanionic materials have emerged as promising options, offering stable battery materials with strong covalent bonds and the potential for higher voltage due to the inductive effect. Various polyanions, encompassing phosphates, sulfates, borates, and oxalates, have been extensively explored in the quest for high-voltage materials. Combining different polyanions together with transition metals to form mixed polyanionic materials presents an appealing opportunity to achieve higher voltage and rapid redox processes [Senthilkumar, B.; Murugesan, C.; Sharma, L.; Lochab. S.; Barpanda, B.; An overview of mixed polyanionic cathode materials for sodium-ion batteries. Small Methods 2019, 3 (4), 1800253-1800276]. Materials containing polyanions like XO-4n (X=Si, P, S) provide larger sites in the structure for alkali metal ions to intercalate, concurrently yielding a higher redox potential. NASICON-type materials are known for their rapid ion conduction and offer the potential to host two alkali metal ions with each formula unit, thereby providing increased theoretical capacity [Goodenough, J. B.; Hong, H.-P.; Kafalas, J., Fast Na+-ion transport in skeleton structures. Mater. Res. Bull. 1976, 11 (2), 203-220]. Wide range of chemical compositions of NASICON-type materials is possible with the presence of highly mobile cations and different synthesis methods are available as well [Anantharamulu, N.; Koteswara Rao, K.; Rambabu, G.; Vijaya Kumar, B.; Radha, V.; Vithal, M., A wide-ranging review on Nasicon type materials. J. Mater. Sci. 2011, 46 (9), 2821-2837]. The Fe3+/Fe2+ redox pair stands out as the most appealing choice for mixed polyanionic materials. In a report by shiva et al., the Fe based mixed polyanionic material is suggested to be a potential cathode material for sodium ion battery [Shiva, K.; Singh, P.; Zhou, W.; Goodenough, J. B., NaFe2PO4(SO4)2: a potential cathode for a Na-ion battery. Energy Environ. Sci. 2016, 9 (10), 3103-3106)]. This preference is attributed to the abundant and cost-effective nature of iron, along with its non-toxic properties. However, such materials have not been explored for lithium ion batteries owing to (a) slow diffusion of Li ions within the solid electrode and electrolyte, resulting in lesser charge storage (b) limited energy density. Further, there exists a room for exploration of efficient electrode materials for lithium ion batteries.
OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide an Fe-based mixed polyanionic electrode material having NASICON-type structure for lithium-ion batteries.

Another objective of the present invention is to provide a process for the preparation of the Fe-based mixed polyanionic electrode material.

Another objective of the present invention is to utilize the material as cathode in lithium 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 Fe-based mixed polyanionic electrode material for lithium ion battery, having a general formula NaFe2(XO4)3, comprising a polyanion XO-n4; wherein n has a value of 2 and 3, wherein the Fe-based mixed polyanionic electrode material has sodium (Na) super ionic conductor (NASICON)-type structure.

The present invention also provides a process for the preparation of an Fe-based mixed polyanionic electrode material as defined above, the process comprises:
i. loading an aqueous solution of a phosphate precursor and an aqueous solution of a sulfate precursor in a feeder of a spray drying instrument; wherein the feeder has an inlet and an outlet,
ii. blowing a hot air through the inlet into the feeder to obtain a homogenous intermediate complex through the outlet; and
iii. annealing the homogenous intermediate complex to obtain the Fe-based mixed polyanionic electrode material.

The present invention provides a cathode for lithium ion battery comprising an Fe-based mixed polyanionic electrode material as defined above.

The present invention provides a lithium ion battery characterize in comprising a cathode comprising an Fe-based mixed polyanionic electrode 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 powder X-ray diffraction (PXRD) patterns of product phases obtained after annealing the spray drying synthesized intermediate complex at different temperatures. Crystalline peaks for NaFe2PO4(SO4¬)¬2 material were observed even at a low calcination temperature of 100 ?C.
Figure 2 depicts structural analysis of spray drying synthesized NaFe2PO4(SO4)2 material (a) Rietveld refinement of the X-ray powder pattern of NaFe2PO4(SO4)2 with space group R-3c with RF = 8.478, ?2 = 2.94. The observed data points (obs.), calculated pattern (calc.), the difference (diff.), and reference (ref.) are shown in red, black, violet, and green, respectively. (b) An illustration of the crystal structure consisting of several lantern units joined together along the b-axis is shown. A single lantern unit consisting of two FeO6 octahedra (pink) joined by two SO4 and one PO4 tetrahedra (blue) along the a, b, and c-axis is also shown.
Figure 3 depicts Rietveld refinement of the neutron powder diffraction (NPD) of NaFe2PO4(SO4)2 with space group R-3c with RF = 3.67, ?2 = 6.65. The observed data points (obs.), calculated pattern (calc.), the difference (diff.), and reference (ref.) are shown in light blue, black, violet, and green, respectively.
Figure 4 depicts (a) Mössbauer spectra for NaFe2PO4(SO4)2 material at 300 K. Fitted spectra black curve for the Fe sub-spectra, which is related to the Fe+3 ions. Corresponding hyperfine parameters are required for better fitting of the observed experimental data (red dots). (b) Quadrupole distribution fit for the doublet spectra obtained. QS denotes quadrupole split.
Figure 5 depicts galvanostatic (dis)charge curves for NaFe2PO4(SO4)2 material at C/20 rate for seven cycles. (b) Cyclic voltammogram at a scan rate of 0.05 mV/s, showing the Fe redox happening during the intercalation process. (c) Rate capability of NaFe2PO4(SO4)2 material at various C-rates form C/20 to 1C upto 45 cycles. (d) Capacity retention at C/10 rate for 50 cycles.
Figure 6 depicts titration techniques for the NaFe2PO4(SO4¬)¬2 material vs Li (a) Potentiostatic intermittent titration technique (PITT) at C/50 rate. (b) Galvanostatic intermittent titration technique (GITT) at C/50. Inset shows the variation of diffusion coefficient with respect to change in voltage, determined using GITT.
Figure 7 depicts in-situ XRD patterns of the NaFe2PO4(SO4)2 material for (dis)charging from a voltage range of 2.0 V to 4.5 V for elucidating the mechanism of Li-ion movement inside the material.

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.

An Fe-based mixed polyanionic electrode material having NASICON-type structure, specifically with the combination of sulfate and phosphate polyanions is disclosed in the present invention. To overcome the limitations of using said material in lithium ion batteries, the Fe-based mixed polyanionic electrode material is prepared using a spray drying technique that offers scalability to material synthesis, resulting in spherical nanoparticles. The nanoscale morphologies offer better diffusion in system. This synthesis route led to the formation of phase-pure and crystalline end-product at a calcination temperature as low as 100 ºC, streamlining its synthesis process and rendering it more conducive to large-scale production. Leveraging the advantages of the nanostructured morphology and spherical particles, Fe-based mixed polyanionic electrode material have been demonstrated to be used as a cathode for LIBs for the first time. Analysis using GITT measurements revealed significantly improved lithium-ion diffusion attributed to the smaller particle size and nanostructured morphology.

Specifically, the present invention provides an Fe-based mixed polyanionic electrode material for lithium ion battery, having a general formula NaFe2(XO4)3, wherein XO-n4 is a polyanion; wherein n has a value of 2 and 3, wherein the Fe-based mixed polyanionic electrode material has sodium (Na) super ionic conductor (NASICON)-type structure.

In the Fe-based mixed polyanionic electrode material, wherein X in the polyanion XO-n4 consist of phosphorous (P) and sulfur (S).

In an embodiment of the present invention, the Fe-based mixed polyanionic electrode material is NaFe2PO4(SO4)2 phosphosulfate.

The Fe-based mixed polyanionic material consisting of spherical nanoparticles having a particle size in a range of 200 to 400 nm.

The specific advantage of employing NaFe2PO4(SO4)2 as a cathode material for LIBs lies in its unique composition as a mixed polyanionic electrode material, comprising phosphate (PO4) and sulfate (SO4) polyanions. In contrast to the oxide-based materials typically utilized in LIB cathodes, mixed polyanionic materials offer enhanced electrochemical performance due to the inductive effect of diverse polyanion units.

The present invention also provides a process for the preparation of an Fe-based mixed polyanionic electrode material as defined above, the process comprises:
i. loading an aqueous solution of a phosphate precursor and an aqueous solution of a sulfate precursor in a feeder of a spray drying instrument; wherein the feeder has an inlet and an outlet,
ii. blowing a hot air through the inlet into the feeder to obtain a homogenous intermediate complex through the outlet; and
iii. annealing the homogenous intermediate complex to obtain the Fe-based mixed polyanionic electrode material.

In an embodiment of the present invention, the phosphate precursor is NaH2PO4.H2O, and the sulfate precursor is FeSO4.7H2O. The NaH2PO4.H2O to FeSO4.7H2O has a mole ratio of 1:2.

The inlet of the feeder is maintained at a temperature in a range of 180 to 220 ? and the outlet of the feeder is maintained at a temperature in a range of 70 to 80 ?.

The homogenous intermediate complex deposited over the substrate is annealed at a temperature in a range of 100 to 500 ? for 12 to 14 hours.

The present invention provides a cathode for lithium ion battery comprising an Fe-based mixed polyanionic electrode material as defined above.

The present invention provides a lithium ion battery characterize in comprising a cathode comprising an Fe-based mixed polyanionic electrode material as defined above.

In an embodiment of the present invention, the lithium ion battery comprises:
i. an anode;
ii. a cathode comprising an Fe-based mixed polyanionic electrode material as defined above; and
iii. an electrolyte.

The Fe-based mixed polyanionic electrode material of the lithium ion battery has a chemical diffusion coefficient in a range of 10-6 to 10-10 cm2/s for lithium ion (de)intercalation.

The cathode of the lithium ion battery has a working potential in a range of 2.0 to 4.5 V.

The anode of the lithium ion battery of the present invention is lithium foil, the electrolyte is selected from 1 M LiPF6 in EC:PC:DMC electrolyte.

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

The Fe-based mixed polyanionic material NaFe2PO4(SO4)2 as a novel cathode material for rechargeable lithium ion batteries is found to be working at an average voltage of 3.2 V with a specific capacity of about 100 mAh g-1 at C/20 rate, with the involvement of Fe3+/Fe2+ redox.

The lithium ion battery of the present invention has good capacity retention of 85% at the end of 50 cycles with close to 100% coulombic efficiency.

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.

Example 1: Preparation of Fe-based mixed polyanionic electrode material NaFe2PO4(SO4)2 phosphosulfate
The NASICON-type NaFe2PO4(SO4)2 phosphosulfate was synthesized using a spray drying process. An aqueous solution of the initial precursors (NaH2PO4.H2O and FeSO4.7H¬2O; 1:2 mole ratio) was put inside the feeder of the spray drying instrument and hot air proceeded into the chamber via the inlet. The procedure was conducted with the inlet temperature at 220 °C and outlet one at 80 °C. The hot air going inside rapidly atomizes the precursor liquid, which is subsequently converted into a homogenous intermediate that came out through the outlet. The atomic scale mixing of the precursors is ensured by the process and the ionicity of the product is kept intact. The whole process happening in a liquid medium makes the diffusion of ions facile compared to other synthesis methods. The whole process involves a two-step process, the first step being the formation of the intermediate complex within the chamber of the spray drying machine. The following second step is annealing the obtained complex for a time duration of 12 h to get the desired product. Also, the end product obtained by the spray drying process generally yields spherically denser particles, which is useful to yield efficient electrochemical performance.

The spray drying intermediate obtained after the synthesis process was annealed at higher temperatures sequentially, and corresponding X-ray diffraction (XRD) patterns were taken as shown in Figure 1. The changes happening in the various peaks obtained with respect to the annealing temperature can be clearly seen in Figure 1. Interestingly, the peaks corresponding to the NaFe2PO4(SO4)2 material are obtained at a temperature as low as 100 ºC, without the presence of any impurity or any other peak. However, the intensity of peaks is lower, which increases as the annealing temperature is increased further up to 500 ºC. This interesting result is different from what already has been reported, with major peaks usually appearing at higher temperatures. It can be due to the intimate mixing of the precursors in the spray drying machine, resulting in the formation of a product and achieving crystallinity, even at a lower annealing temperature of 100 ºC.

Inset of Figure 2 shows the scanning electron microscopy (SEM) image of the obtained product. Spherically shaped and nanometric-sized particles were produced through the spray drying synthesis of materials. The spherical shape can be attributed to the physics and mechanism involved in the spray drying process. By adjusting parameters such as the size of the droplets, the flow rate, and the drying conditions, the final particle size can be controlled as well with desired spherical shape.

Crystal Structure:
The NaFe2PO4(SO4¬)¬2 material has a NASICON-type structure with FeO6 octahedra and XO4 tetrahedra (X: S, P). Phosphate and sulfate tetrahedra are present at identical positions in the Fe2(XO4)3 skeleton units (lantern units), connecting the two FeO6 octahedra via corner-sharing. 3D interconnected channels are present with sodium atoms and vacancies distributed in two different types of interstitial positions. The iron (Fe) atom is coordinated with six oxygen (O) atoms to form a FeO6 octahedron. The FeO6 octahedra and PO4/SO4 tetrahedra are interconnected via shared oxygen atoms to create a three-dimensional (3D) framework, and the sodium (Na) atoms present in the interstitial spaces of the framework, surrounded by six oxygen atoms, as shown in the Figure 2b. To confirm the structural and phase purity, X-ray diffraction (XRD) and neutron powder diffraction (NPD) of the NaFe2PO4(SO4¬)¬2 material were obtained. Rietveld refinement of both XRD and NPD patterns was done using NaTi2(PO4)3 as the starting model (Figure 2a, Figure 3).26 The structure with R-3c space group matched with earlier reports of NaFe2PO4(SO4)2.26,27

Mössbauer spectroscopy was employed to gain insights into the local chemical environment around Fe in the NaFe2PO4(SO4¬)¬2 material (Figure 4). The Mössbauer spectrum was successfully fitted using a quadrupole doublet, indicating a non-magnetic environment around Fe at 300 K (Figure 4a). The isomer shift and quadrupole split values indicate that Fe is solely present in a 3+ oxidation state with octahedral coordination. The obtained doublet spectra are symmetric in nature and can thus be best described using a quadrupole distribution fit (Figure 4b). It indicates two different Fe environments with very similar isomer shifts but slightly different quadrupole splits. The ratio of these two distributions is close to 1:2, which is similar to that of the polyanion ligand ratio, (PO4)3-:(SO4)2- for the NaFe2PO4(SO4¬)¬2 material. This indicates that although all the Fe3+ ions occupy one crystallographic sub-lattice, their local chemical environment is slightly different depending on the coordinating ligand.

Electrochemical properties:
The electrochemical activity of the Fe-based mixed polyanionic electrode material NaFe2PO4(SO4)2 was studied in a half-cell configuration for lithium-ion battery, with Li metal foil as negative electrode using CR2032 coin cells. For lithium-ion battery configuration, galvanostatic charge-discharge (GCD) was carried out in the potential range of 2.0 - 4.5 V with 1 M LiPF6 in EC:PC:DMC electrolyte (Figure 5a). At a current rate of C/20, reversible voltage profiles were obtained with a slopy profile, with an average voltage of 3.2 V and discharge capacity of 99 mAh/g (1.56 Li+, theoretical capacity: 127 mAh/g for 2e- reaction). The obtained capacity accounts for 78% of the theoretical capacity, and the slopy profile suggests a single-phase reaction mechanism. The following (dis)charge cycles showed the same slopy profile. The cyclic voltammograms (CV) confirm the presence of Fe+3/Fe+2 redox behavior (Figure 5b). Considering the good discharge capacity obtained, rate capability at different current rates of C/20 (0.05C), C/10 (0.1 C), C/5 (0.2 C), C/2 (0.5 C) and 1C were obtained till 45 cycles, resulting in discharge capacities of 96 mAh/g, 89 mAh/g, 82 mAh/g, 76 mAh/g, 64 mAh/g (Figure 5c). At a faster rate of 1C, lesser discharge capacity was obtained, probably because of lower electronic conductivity. Figure 5d shows the cycling stability of the material at C/10 rate for 50 cycles. A discharge capacity of 91 mAh/g is obtained in the first cycle which fades upto 77 mAh/g in the 50th cycle. So, good capacity retention of 85% at the end of 50 cycles is observed with close to 100% coulombic efficiency.

To understand the redox process of lithium-ion (de)insertion, we employed electrochemical titration methods, specifically, galvanostatic intermittent titration technique (GITT) and potentiostatic intermittent titration technique (PITT), conducted at a slower rate of C/50, with a one-hour rest period for each titration step (Figure 6). PITT measurements showed a current response characteristic of a dominant solid-solution redox mechanism with some minor structural changes occurring in the material as well. GITT measurements were used to examine the chemical diffusion kinetics of lithium metal ions in the material by monitoring the variations in lithium metal ion content. This allows us to assess the transport of lithium ions, which follows Fick's second law of diffusion. The diffusion coefficient was determined assuming a constant molar volume (VM) despite the variation in the concentration of alkali metal ion during (de)intercalation. For Li ion (de)intercalation, the chemical diffusion coefficient ranged from 10-6 to 10-10 cm2/s.

Lithium ion (De)insertion Mechanism:
For studying the underlying mechanism of electrochemistry for the (de)insertion of NaFe2PO4(SO4)2 material, in-situ XRD analysis was performed for Li-ion battery configuration. In-situ XRD measurements were performed in an in-situ electrochemical cell with NaFe2PO4(SO4)2 as the positive electrode, fitted within a beryllium window in a Bruker D8 diffractometer. In-situ XRD helps in gaining insights into how the crystal structure of the material evolves with respect to the insertion and extraction of lithium (Li+) ions. Figure 7 depicts the in-situ XRD data for the Li-ion configuration for the NaFe2PO4(SO4)2 material. The cell was electrochemically cycled at C/35 while the XRD patterns were continuously recorded. Notably, there is a continuous and noticeable shift observed in the prominent peaks corresponding to the (113) and (116) crystallographic planes. The enlarged version of the XRD pattern provides a clearer view of these shifts. Although the shift is more significant for the (116) peak than the (113) peak. As the Li+ ion insertion process occurs during the discharge cycle, the peaks progressively move towards lower 2? values without the appearance of any new peaks. This behavior is consistent with the characteristic pattern of one-phase evolution, signifying a seamless transition. A slight change in the lattice parameters was observed as well with the amount of inserted Li+. Conversely, during the Li+ ion extraction process, the peaks undergo a reverse shift, eventually returning to their initial positions. This observation serves as strong evidence for the complete reversibility of the process. So, from above observations, it can be concluded that a single-phase evolution can be observed for the NaFe2PO4(SO4)2 material in the Li-ion battery configuration.

Advantages of the present invention over the prior art.
• The present invention provide NASICON type NaFe2PO4(SO4)2 phosphosulfate as an economic Fe-based 3.2 V cathode material for Li-ion battery having an excellent reversible discharge capacity of about 100 mAh/g.
• NaFe2PO4(SO4)2 is cost-effective and can be readily synthesized. Compared to oxide cathode materials, it shows moderate energy density. Still, it is well-suited for affordable small electronic devices and stationary energy storage solutions.
• NaFe2PO4(SO4)2 material of the present invention is synthesized at a lower annealing temperature of 100 ºC. Typical annealing temperature for the synthesis of oxides and polyanionic materials, especially involving sulfate polyanionic groups, is above 500 ºC.
• The lithium ion battery of the present invention has (i) cathode comprising economically accessible elements (Fe, P, S, O), (ii) used spray drying process to create nanoscale morphologies and pure phase at a very low calcination temperature of 100 °C, (iii) a reversible capacity of about100 mAh/g at a current rate of C/20 with a theoretical capacity of 127 mAh/g (involving a Fe3+/Fe2+ redox potential at 3.2 V vs. Li+/Li).
• The specific advantage of employing NaFe2PO4(SO4)2 as a cathode material for LIBs lies in its unique composition as a mixed polyanionic material, comprising phosphate (PO4) and sulfate (SO4) polyanions. In contrast to the oxide-based materials typically utilized in LIB cathodes, mixed polyanionic materials offer enhanced electrochemical performance due to the inductive effect of diverse polyanion units. , Claims:1. An Fe-based mixed polyanionic electrode material for lithium ion battery, having a general formula NaFe2(XO4)3, wherein XO-n4 is a polyanion; wherein n has a value of 2 and 3; wherein the Fe-based mixed polyanionic material has sodium (Na) super ionic conductor (NASICON)-type structure.

2. The Fe-based mixed polyanionic electrode material as claimed in claim 1, wherein X in the polyanion XO-n4 comprises phosphorous (P) and sulfur (S).

3. The Fe-based mixed polyanionic electrode material as claimed in claim 1, wherein the Fe-based mixed polyanionic material is NaFe2PO4(SO4)2 phosphosulfate.

4. The Fe-based mixed polyanionic electrode material as claimed in claims 1- 3, wherein the Fe-based mixed polyanionic material has a particle size in a range of 200 to 400 nm.

5. A process for the preparation of an Fe-based mixed polyanionic electrode material as claimed in claims 1-4, the process comprises:
i. loading an aqueous solution of a phosphate precursor and an aqueous solution of a sulfate precursor in a feeder of a spray drying instrument; wherein the feeder has an inlet and an outlet,
ii. blowing a hot air through the inlet into the feeder to obtain a homogenous intermediate complex through the outlet; and
iii. annealing the homogenous intermediate complex to obtain the Fe-based mixed polyanionic electrode material.

6. The process as claimed in claim 5, wherein the phosphate precursor is NaH2PO4.H2O, and the sulfate precursor is FeSO4.7H2O.

7. The process as claimed in claims 5 and 6, wherein the NaH2PO4.H2O to FeSO4.7H2O has a mole ratio of 1:2.

8. The process as claimed in claim 5, wherein the inlet is maintained at a temperature in a range of 180 to 220 to ? and the outlet is maintained at a temperature in a range of 70 to 80 ?.

9. The process as claimed in claim 5, wherein the annealing is performed at a temperature in a range of 100 to 500 ? for 12 to 14 hours.

10. A cathode for lithium ion battery comprising an Fe-based mixed polyanionic electrode material as claimed in claims 1-4.

11. A lithium ion battery characterize in comprising a cathode comprising an Fe-based mixed polyanionic electrode material as claimed in claims 1-4.

12. The lithium ion battery as claimed in claim 11, wherein the Fe-based mixed polyanionic electrode material has a chemical diffusion coefficient in a range of 10-6 to 10-10 cm2/s for lithium ion (de)intercalation.

13. The lithium ion battery as claimed in claim 11, wherein the cathode has a working potential in a range of 2.0 to 4.5 V.