1. An electrolyte formulation comprising: (a) at least one sodium salt; (b) a combination of solvents; and (c) at least four additives selected from the group consisting of: (i) at least one nitrile compound; (ii) fluoroethylene carbonate; (iii) at least one sulfur containing compound; and (iv) at least one fluorine containing compound, wherein the weight ratio of (i) to (iv) is in the range of 0.5:0.5:0.5:1 to 2:5:3:1.

2. The electrolyte formulation as claimed in claim 1, wherein the nitrile compound has a weight percentage in the range of 1 to 2%.

3. The electrolyte formulation as claimed in claim 1, wherein the fluoroethylene carbonate has a weight percentage in the range of 1-5%.

4. The electrolyte formulation as claimed in claim 1, wherein the sulphur containing compound has a weight percentage in the range of 1-3%.

5. The electrolyte formulation as claimed in claim 1, wherein the fluorine containing compound has a weight percentage in the range of 1-2%.

6. The electrolyte formulation as claimed in claim 1 or 2, wherein the nitrile compound is selected from the group consisting of succinonitrile, adiponitrile, and glutaronitrile.

7. The electrolyte formulation as claimed in claim 1 or 4, wherein the sulfur containing compound is selected from the group consisting of 1,3 Propane sultone (1,3 PS), glycol sulfite (ES), dimethyl sulfite (DMS), propylene sulfate (PSO4), and 1,3- propylene sulfite (1,3-PSF).

8. The electrolyte formulation as claimed in claim 1 or 5, wherein the fluorine containing compound is selected from the group consisting of sodium difluoro(oxalato)borate (NaDFOB), sodium hexafluorophosphate, sodium bis(oxala to)borate (NaBOB), sodium tetrafluoroborate (NaBF4), sodium bis(trifluoromethanesulfonyl)imide (NaNTf), sodium trifluoromethanesulfonate, sodium tetrachlorogallate, sodium tetrachloroaluminate, and sodium hexafluoroarsenate (V).

9. The electrolyte formulation as claimed in claim 1, wherein the sodium salt is selected from the group consisting of sodium hexafluorophosphate (NaPF6), sodium difluo-ro(oxalato)borate (NaDFOB), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (V) (NaAsF6), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).

10. The electrolyte formulation as claimed in claim 1 or 9, wherein the sodium salt has a concentration in the range of 0.8 M to 1.5 M.

11. The electrolyte formulation as claimed in claim 1, wherein the combination of the solvents is selected from the group consisting of ethylene carbonate (EC), butylene carbonate (BC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EM).

12. The electrolyte formulation as claimed in claim 1 or 11, wherein the combination of solvents is selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and wherein ethylene carbonate (EC), propyl-ene carbonate (PC), dimethyl carbonate (DMC) has a volume ratio in the range of 0.5:1:0.5-1:1.5:2.

13. The electrolyte formulation as claimed in anyone of the claims 1 to 12, wherein the electrolyte has a stability in the range of 5.6 Volts-6 Volts.

14. A method for preparing an electrolyte formulation, said method comprising the steps of: (a) obtaining a combination of solvents; (b) adding a sodium salt to the combination of solvents at a temperature in the range of 27 to 37°C to obtain a first mixture; and (c) adding at least one nitrile, fluoroethylene carbonate, at least one sulfur containing compound, and at least one fluorine containing compound simultaneously to the said first mixture, followed by mixing to obtain the electrolyte formulation.

15. The method as claimed in claim 14, wherein the combination of solvents is selected from the group consisting of ethylene carbonate (EC), butylene carbonate (BC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EM), and wherein the sodium salt is selected from the group consisting of sodium hexafluorophosphate (NaPF6), sodium difluoro(oxalato)borate (NaDFOB), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (V) (NaAsF6), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).

16. The method as claimed in claim 14, wherein the nitrile compound is selected from the group consisting of succinonitrile, adiponitrile, and glutaronitrile.

17. The method as claimed in claim 14, wherein the sulfur containing compound is selected from the group consisting of 1, 3 Propane sultone (1,3 PS), glycol sulfite (ES), dimethyl sulfite (DMS), propylene sulfate (PSO4), and 1,3- propylene sulfite (1,3-PSF).

18. The method as claimed in claim 14, wherein the fluorine containing compound is selected from the group consisting of sodium difluoro(oxalato)borate (NaDFOB), sodium hex-afluorophosphate, sodium bis(oxala to)borate (NaBOB), sodium tetrafluoroborate (NaBF4), sodium bis(trifluoromethanesulfonyl)imide (NaNTf), sodium trifluoro-methanesulfonate, sodium tetrachlorogallate, sodium tetrachloroaluminate, and sodium hexafluoroarsenate (V).

19. The method as claimed in claim 14, wherein adding the at least one nitrile, the fluoroeth-ylene carbonate, the at least one sulfur containing compound, and the at least one fluorine containing compound to the said first mixture is done at a temperature in the range of 27°C to 37°C, and wherein mixing in step (c) is carried out at a temperature in the range of 27°C to 37°C for a time period in the range of 10hr to 24hr.

20. A sodium-ion battery comprising the electrolyte formulation as claimed in anyone of the claims 1 to 13.

We Claim:
1. An electrolyte formulation comprising:
(a) at least one sodium salt;
(b) a combination of solvents; and
(c) at least four additives selected from the group consisting of: (i) at least one nitrile compound;
(ii) fluoroethylene carbonate;
(iii) at least one sulfur containing compound; and (iv) at least one fluorine containing compound, wherein the weight ratio of (i) to (iv) is in the range of 0.5:0.5:0.5:1 to 2:5:3:1.
2. The electrolyte formulation as claimed in claim 1, wherein the nitrile compound has a weight percentage in the range of 1 to 2%.
3. The electrolyte formulation as claimed in claim 1, wherein the fluoroethylene carbonate has a weight percentage in the range of 1-5%.
4. The electrolyte formulation as claimed in claim 1, wherein the sulphur containing compound has a weight percentage in the range of 1-3%.
5. The electrolyte formulation as claimed in claim 1, wherein the fluorine containing compound has a weight percentage in the range of 1-2%.
6. The electrolyte formulation as claimed in claim 1 or 2, wherein the nitrile compound is selected from the group consisting of succinonitrile, adiponitrile, and glutaronitrile.
7. The electrolyte formulation as claimed in claim 1 or 4, wherein the sulfur containing compound is selected from the group consisting of 1,3 Propane sultone (1,3 PS), glycol sulfite (ES), dimethyl sulfite (DMS), propylene sulfate (PSO4), and 1,3- propylene sulfite (1,3-PSF).
8. The electrolyte formulation as claimed in claim 1 or 5, wherein the fluorine containing compound is selected from the group consisting of sodium difluoro(oxalato)borate (NaDFOB), sodium hexafluorophosphate, sodium bis(oxala to)borate (NaBOB), sodium tetrafluoroborate (NaBF4), sodium bis(trifluoromethanesulfonyl)imide (NaNTf), sodium trifluoromethanesulfonate, sodium tetrachlorogallate, sodium tetrachloroaluminate, and sodium hexafluoroarsenate (V).
9. The electrolyte formulation as claimed in claim 1, wherein the sodium salt is selected from the group consisting of sodium hexafluorophosphate (NaPF6), sodium difluo-ro(oxalato)borate (NaDFOB), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (V) (NaAsF6), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).
10. The electrolyte formulation as claimed in claim 1 or 9, wherein the sodium salt has a concentration in the range of 0.8 M to 1.5 M.
11. The electrolyte formulation as claimed in claim 1, wherein the combination of the solvents is selected from the group consisting of ethylene carbonate (EC), butylene carbonate (BC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EM).
12. The electrolyte formulation as claimed in claim 1 or 11, wherein the combination of solvents is selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and wherein ethylene carbonate (EC), propyl-ene carbonate (PC), dimethyl carbonate (DMC) has a volume ratio in the range of 0.5:1:0.5-1:1.5:2.
13. The electrolyte formulation as claimed in anyone of the claims 1 to 12, wherein the electrolyte has a stability in the range of 5.6 Volts-6 Volts.
14. A method for preparing an electrolyte formulation, said method comprising the steps of:
(a) obtaining a combination of solvents;
(b) adding a sodium salt to the combination of solvents at a temperature in the range of 27 to 37°C to obtain a first mixture; and
(c) adding at least one nitrile, fluoroethylene carbonate, at least one sulfur containing compound, and at least one fluorine containing compound simultaneously to the said first mixture, followed by mixing to obtain the electrolyte formulation.
15. The method as claimed in claim 14, wherein the combination of solvents is selected from the group consisting of ethylene carbonate (EC), butylene carbonate (BC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EM), and wherein the sodium salt is selected from the group consisting of sodium hexafluorophosphate (NaPF6), sodium difluoro(oxalato)borate (NaDFOB), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (V) (NaAsF6), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).
16. The method as claimed in claim 14, wherein the nitrile compound is selected from the group consisting of succinonitrile, adiponitrile, and glutaronitrile.
17. The method as claimed in claim 14, wherein the sulfur containing compound is selected from the group consisting of 1, 3 Propane sultone (1,3 PS), glycol sulfite (ES), dimethyl sulfite (DMS), propylene sulfate (PSO4), and 1,3- propylene sulfite (1,3-PSF).
18. The method as claimed in claim 14, wherein the fluorine containing compound is selected from the group consisting of sodium difluoro(oxalato)borate (NaDFOB), sodium hex-afluorophosphate, sodium bis(oxala to)borate (NaBOB), sodium tetrafluoroborate (NaBF4), sodium bis(trifluoromethanesulfonyl)imide (NaNTf), sodium trifluoro-methanesulfonate, sodium tetrachlorogallate, sodium tetrachloroaluminate, and sodium hexafluoroarsenate (V).
19. The method as claimed in claim 14, wherein adding the at least one nitrile, the fluoroeth-ylene carbonate, the at least one sulfur containing compound, and the at least one fluorine containing compound to the said first mixture is done at a temperature in the range of 27°C to 37°C, and wherein mixing in step (c) is carried out at a temperature in the range of 27°C to 37°C for a time period in the range of 10hr to 24hr.
20. A sodium-ion battery comprising the electrolyte formulation as claimed in anyone of the claims 1 to 13.