Description:FIELD OF THE INVENTION
The present invention relates to a mixed alkali metal polyanionic compound for battery applications. More specifically, the present invention relates to a mixed alkali metal polyanionic compound and a process for preparation thereof. The invention further relates to a battery electrode comprising the mixed alkali metal polyanionic compound and a process for the preparation of the electrode for sodium ion batteries.

BACKGROUND OF THE INVENTION
Sodium ion batteries (SIBs) have garnered significant attention as potential alternatives to lithium-ion batteries due to the abundance and low cost of sodium resources. However, the performance and stability of SIBs still need improvement to compete effectively with lithium-ion batteries. In order to provide performance on par with lithium technology, it is important to engineer materials which can provide excellent capacity and voltage thus enabling better energy and power density as lithium-ion battery technology.

Polyanionic compounds have emerged as promising electrode materials for SIBs due to their structural stability, high voltage and high specific capacities. Among the various polyanionic compounds, Na3V2(PO4)2O2F, Na3V2(PO4)2F3 and Na3V2(PO4)3 gained enormous attention. However, challenges such as poor electronic conductivity and sluggish sodium ion diffusion kinetics hinder their practical application.

CN108417792A discloses the preparation methods of high-performance aluminum potassium codope fluorophosphoric acid vanadium sodium/carbon composite with generic formula Na1-xKxV1-yAlyPO4F/C for battery application. The material is synthesized using chemical synthesis techniques, followed by a high-temperature treatment conducted in an inert atmosphere. While the co-doped sample exhibits commendable battery performance, the additional thermal treatment requirement contributes to elevated costs. Furthermore, the material's demonstrated cycling capability is limited to 100 cycles, underscoring the necessity for battery materials with superior cycling stability under cost effective synthesis conditions.

OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide a mixed alkali metal polyanionic compound for battery applications.

Another objective of the present invention is to provide a process for the preparation of the mixed alkali metal polyanionic compound.

Another objective of the present invention is to provide an electrode comprising the mixed alkali metal polyanionic compound.

Another objective of the present invention is to provide a process for the preparation of the electrode comprising the mixed alkali metal polyanionic compound.

Another objective of the present invention is to utilize the electrode comprising the mixed alkali metal polyanionic compound as cathode 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 a mixed alkali metal polyanionic compound (M-PAC) represented by a general formula AaB1-bC1-a-bDcEx-eFx-e-f(PO4)x, wherein A, B and C is an alkali metal cation; D is a transition metal; E is a non-metal, and F is a halogen;
wherein PO4 has a polyanionic framework;
wherein ‘a’ is in a range from 0 to 3.1, ‘b’ is in a range from 0.01 to 0.5, ‘c’ is in a range from 0.01 to 0.5, and e, f & x is in a range from 0 to 2 based on the stoichiometry of the mixed alkali polyanionic compound.

The present invention also provides a process for the preparation of a mixed alkali metal polyanionic compound as defined above, said process comprising:
i. mixing a transition metal precursor with oxalic acid in water to form a solution;
ii. adding a phosphate (PO4) precursor, and an alkali metal precursor to the solution;
iii. heating the solution to obtain a product; and
iv. washing the product with a solvent and drying the product to obtain the mixed alkali polyanionic compound.

The present invention also provides an electrode comprising a mixed alkali metal polyanionic compound as defined above for sodium ion battery.

The present invention further provides a process for the preparation of an electrode comprising a mixed alkali metal polyanionic compound as defined above, the process comprising:
i. mixing 70 to 98 weight % of the mixed alkali metal polyanionic compound with a conductive additive and a binder to form a slurry; and
ii. depositing the slurry over a substrate to prepare the electrode.

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 XRD pattern of bare-polyanionic compound (Bare-PAC) and the mixed alkali metal polyanionic compound (M-PAC).
Figure 2 depicts SEM images of (a) Bare-PAC and (b) M-PAC at different magnifications.
Figure 3 depicts charge-discharge profile of the M-PAC at a rate of C/5.
Figure 4 depicts rate capability comparison of both Bare-PAC and M-PAC samples at different rates from C/5-C/2-1C and 2C.
Figure 5 depicts cycling stability of the M-PAC at 1C for 1000 cycles with 96% retention.

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.

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.

The present invention offers an advance solution to the challenges posed by conventional battery materials, thus significantly enhancing the performance and cost-effectiveness of battery technologies by introducing unique methodologies that optimize cycling stability while minimizing synthesis complexities.

The present invention provides a mixed alkali metal polyanionic compound (M-PAC) represented by a general formula AaB1-bC1-a-bDcEx-eFx-e-f(PO4)x, wherein A, B and C is an alkali metal cation; D is a transition metal; E is a non-metal, and F is a halogen;
wherein PO4 has a polyanionic framework;
wherein ‘a’ is in a range from 0 to 3.1, ‘b’ is in a range from 0.01 to 0.5, ‘c’ is in a range from 0.01 to 0.5, and e, f & x is in a range from 0 to 2 based on the stoichiometry of the mixed alkali polyanionic compound.

In an embodiment of the present invention, the alkali metal cation is selected from lithium (Li), sodium (Na) and potassium (K), the transition metal is vanadium (V), the non-metal is selected from a group comprising oxygen, sulphur, selenium, and tellurium; and the halogen is fluorine (F).

In an embodiment of the present invention, the alkali metal cation A is sodium (Na), B is potassium (K), and C is lithium, although other mixed alkali combinations can also be utilized. The mixed alkali cations are incorporated into the polyanionic framework, providing additional ion diffusion pathways and improving the overall electrochemical performance of the compound.

In an embodiment of the present invention, the mixed alkali metal polyanionic compound of the present invention has a particle size in a range of 100 to 400 nm with a faceted assembly.

The alkali metal cation in mixed alkali metal polyanionic compound is incorporated into the polyanionic framework of PO4.

The present invention also provides a process for the preparation of a mixed alkali metal polyanionic compound as defined above, said process comprising:
i. mixing a transition metal precursor with oxalic acid in water to form a solution;
ii. adding a phosphate (PO4) precursor, and an alkali metal precursor to the solution;
iii. heating the solution to obtain a product; and
iv. washing the product with a solvent and drying the product to obtain the mixed alkali polyanionic compound.

In an embodiment of the present invention, the transition metal precursor is vanadium pentoxide. The vanadium pentoxide to oxalic acid has a molar ratio of 1:2.5 to 1:3.

In an embodiment of the present invention, the phosphate (PO4) precursor, and the alkali metal precursor are added to the solution in stoichiometric amount predetermined from the composition.

The alkali metal precursor is selected a group comprising sodium carbonate, sodium fluoride, sodium acetate, sodium phosphate monobasic dihydrate, lithium fluoride, lithium hydroxide, lithium phosphate, lithium carbonate, lithium acetate, lithium hexafluorophosphate, potassium hydroxide, potassium carbonate, potassium acetate, potassium hexafluorophosphate, potassium fluoride, and a combination thereof. The phosphate (PO4) precursor is ammonium dihydrogen phosphate.

In an embodiment of the present invention, the reaction mixture is heated through a microwave or a hydrothermal treatment.

In an embodiment of the present invention, the reaction mixture is heated at a temperature in a range of 80 to 240 ? for 10 min to 24 hrs.

The solvent for washing of product is selected from a group comprising water, ethanol, acetone, isopropyl alcohol, and a combination thereof. The water is deionized water.

The product is dried at a temperature in a range of 40 to 120 ?.

The present invention also provides an electrode comprising a mixed alkali metal polyanionic compound as defined above for sodium ion battery.

In an embodiment of the present invention, the electrode is a cathode.

The present invention further provides a process for the preparation of an electrode comprising a mixed alkali metal polyanionic compound as defined above, the process comprising:
i. mixing 70 to 98 weight % of the mixed alkali metal polyanionic compound with a conductive additive and a binder to form a slurry; and
ii. depositing the slurry over a substrate to prepare the electrode.

The conductive additive is selected from a group comprising carbon black, acetylene black, pitch, carbon fibers, carbon nanaotubes, ketjen black, conductive polymers, and a combination thereof.

The conductive additive is added in a range from 0.01 to 15 weight %.

The binder is selected from a group comprising polyvinylpyrrolidone, polyvinylidene fluoride, carboxy methyl cellulose sodium salt, polyacrylic acid sodium salt, cellulose, styrene-butadiene rubber, and a combination thereof.

The substrate is selected from a group consisting of aluminium foil, stainless steel, carbon coated aluminium foil, bipolar plates, copper foil or an electrically conductive material.

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 synthesis of the mixed alkali incorporated polyanionic compound is either via microwave or hydrothermal method, without any post thermal treatment. The synthesis duration ranges from 10 minutes to 24 hours.

Example 1: Synthesis of mixed alkali incorporated polyanionic compound by microwave heating
In the particular example, mix of sodium fluoride and sodium carbonate was used as the precursor for sodium. 1 mol of vanadium pentoxide is dissolved in 2.5 mol of oxalic acid solution in water followed by the addition of stochiometric proportion of ammonium dihydrogen phosphate, sodium precursor along with potassium carbonate and lithium carbonate. The solution is allowed to have proper mixing and later transferred to microwave reactors, setting the reaction conditions at 180? for 30 minutes. After completion, the samples were washed and dried to remove the moisture content. Solvent for washing is either one or mix of deionised water, ethanol, acetone, isopropyl alcohol.

Example 2: Synthesis of mixed alkali incorporated polyanionic compound by hydrothermal heating
In the particular example, mix of sodium fluoride and sodium carbonate was used as the precursor for sodium. Vanadium pentoxide, at a molar ratio of 1:3 with respect to oxalic acid, is dissolved in a solution. Subsequently, this solution is combined with a stoichiometrically determined quantity of ammonium dihydrogen phosphate, along with a sodium precursor, lithium carbonate and potassium carbonate. The resulting solution undergoes thorough mixing and is then transferred to hydrothermal reactors, where reaction conditions are set at 180? for a duration of 5 hrs. Upon completion of the reaction, the resultant samples are subjected to washing with deionized water followed by ethanol washing and further drying at 50°C.

Example 3: Electrode and Cell Fabrication:
The mixed alkali polyanionic compound synthesised either by hydrothermal or microwave synthesis was fabricated as an electrode via slurry casting technique. The mixed alkali polyanionic compound as active component was mixed with conductive additive and binder. The active component ranges from 85 weight %. Carbon black was used as the conductive additive. The content of conductive additives ranges from 10 weight %. Additionally, the binder used was carboxy methyl cellulose in a specific weight percentage of 5 weight %.

The crystal structure of the mixed alkali polyanionic compound (M-PAC) was analysed through X-ray diffraction (XRD). The XRD of M-PAC matches well with the bare polyanionic compound (Bare-PAC) sodium vanadium fluorophosphate without any presence of impurities as shown in figure 1. Morphological characterization in figure 2 confirms the M-PAC has particle size in the range of 100-200 nm. However, incorporation of mixed alkali resulted in Ostwald ripening and associated growth leading to particle size in the range of 300 to 400 nm with faceted assemblies. The material can be engineered to obtain different morphologies ranging from nanoparticles, cuboids, flower assemblies etc. depending on the synthesis conditions.

The electrochemical characterization confirms the improved performance of M-PAC as compared to Bare-PAC. The typical charge-discharge profile of M-PAC at C/5 rate is shown in figure 3 delivering a discharge capacity of 117 mAh/g and coulombic efficiency close to 91%. Though the low rate (C/5) specific capacity is almost comparable (117 and 112 mAh/g), M-PAC delivered high performances at faster rates as demonstrated in figure 4. At a fast rate of 1C and 2C, the M-PAC delivered capacity of 109 and 102 mAh/g while Bare sample capacity was only 93 and 81 mAh/g. M-PAC shows a cycling stability at 1C for 1000 cycles with 91% retention as shown in figure 5. The delivered capacities are respectively around 112 mAh/g and 102 mAh/g at 1st cycle and at the end of 100th cycle.

The advantages of the present invention over the prior art are as follows:

• The present invention provides a process for the preparation of mixed metal ion incorporated sodium vanadium fluorophosphate which is a significant improvement over existing techniques, promising greater efficiency and scalability in production.
• Unlike traditional methods, which may be complex or limited in scalability, the disclosed process is facile and easily scalable, thereby enhancing its practicality and commercial viability.
• Additionally, the material produced exhibits superior electrochemical performance, including enhanced capacity delivery, superior cycling stability, and rapid charging capabilities, surpassing the performance metrics of prior art materials. Moreover, the specific stoichiometry and unique properties of the synthesized material contribute to its unparalleled performance, setting it apart from existing technologies. , Claims:1. A mixed alkali metal polyanionic compound represented by a general formula AaB1-bC1-a-bDcEx-eFx-e-f(PO4)x, wherein A, B and C is an alkali metal cation; D is a transition metal; E is a non-metal, and F is a halogen;
wherein PO4 has a polyanionic framework,
wherein ‘a’ is in a range from 0 to 3.1, ‘b’ is in a range from 0.01 to 0.5, ‘c’ is in a range from 0.01 to 0.5, and e, f & x is in a range from 0 to 2 based on the stoichiometry of the mixed alkali polyanionic compound.

2. The mixed alkali metal polyanionic compound as claimed in claim 1, wherein the alkali metal cation is selected from lithium (Li), sodium (Na) and potassium (K), the transition metal is vanadium (V), the non-metal is selected from a group comprising oxygen, sulphur, selenium, and tellurium; and the halogen is fluorine (F).

3. The mixed alkali metal polyanionic compound as claimed in claim 1, wherein the alkali metal cation A is sodium (Na), B is potassium (K), and C is lithium (Li).

4. The mixed alkali metal polyanionic compound as claimed in claim 1, wherein the alkali metal cation is incorporated into the polyanionic framework of PO4.

5. A process for the preparation of a mixed alkali metal polyanionic compound as claimed in claims 1 to 4, said process comprises:
i. mixing a transition metal precursor with oxalic acid in water to form a solution;
ii. adding a phosphate (PO4) precursor, and an alkali metal precursor to the solution;
iii. heating the solution to obtain a product; and
iv. washing the product with a solvent and drying the product to obtain the mixed alkali metal polyanionic compound.

6. The process as claimed in claim 5, wherein the transition metal precursor is vanadium pentoxide; wherein vanadium pentoxide to oxalic acid has a molar ratio of 1:2.5 to 1:3.

7. The process as claimed in claim 5, wherein the alkali metal precursor is selected a group comprising sodium carbonate, sodium fluoride, sodium acetate, sodium phosphate monobasic dihydrate, lithium fluoride, lithium hydroxide, lithium phosphate, lithium carbonate, lithium acetate, lithium hexafluorophosphate, potassium hydroxide, potassium carbonate, potassium acetate, potassium hexafluorophosphate, potassium fluoride, or a combination thereof; wherein the phosphate (PO4) precursor is ammonium dihydrogen phosphate.

8. The process as claimed in claims 5, wherein the solution is heated at a temperature in a range of 80 to 240 ? for 10 min to 24 hrs through a microwave or a hydrothermal treatment.

9. The process as claimed in claims 5, wherein the solvent for washing is selected from a group comprising water, ethanol, acetone, isopropanol or a combination thereof; wherein the drying is carried out at a temperature in a range of 40 to 120 ?.

10. An electrode comprising a mixed alkali metal polyanionic compound as claimed in claims 1 to 4 for sodium ion battery.

11. The electrode as claimed in claim 10, wherein the electrode is a cathode.

12. A process for the preparation of an electrode comprising a mixed alkali metal polyanionic compound, as claimed in claims 10 and 11, the process comprises:
i. mixing 70 to 98 weight % of the mixed alkali metal polyanionic compound with a conductive additive and a binder to form a slurry; and
ii. depositing the slurry over a substrate to prepare the electrode.

13. The process as claimed in claim 12, wherein the conductive additive is selected from a group comprising carbon black, acetylene black, pitch, carbon fibers, carbon nanaotubes, ketjen black, conductive polymers, and a combination thereof.

14. The process as claimed in claim 12, wherein the conductive additive is added in a range from 0.01 to 15 weight %.

15. The process as claimed in claim 12, wherein the binder is selected from a group comprising polyvinylpyrrolidone, polyvinylidene fluoride, carboxy methyl cellulose sodium salt, polyacrylic acid sodium salt, cellulose, styrene-butadiene rubber, and a combination thereof.