Description:HIGH VOLTAGE ELECTRIC DOUBLE LAYER CAPACITOR (EDLC), ELECTROLYTE AND IMPLEMENTATION THEREOF

FIELD OF INVENTION:

[001] The present invention relates to the field of electrochemical energy storage devices and more specifically relates to a high voltage electric double layer capacitor (EDLC) and an electrolyte present in the said capacitor.

BACKGROUND OF INVENTION:

[002] Supercapacitor cells are considered as the auspicious and proficient energy storage units for different commercial purposes. Mostly, electric double-layer capacitors (EDLCs) are used nowadays due to their substantially higher power density, faster charge-discharge rates, and splendid cyclic stability. But the commercial EDLCs face complications primarily due to their restricted-energy density (as compared to conventional batteries) which ultimately limits their potential/high-end applications. Besides, the working voltage of commercially available supercapacitors cells lies between 2.3-2.7 V. Since it is well-known that the estimated energy density (E) = 1/2 x C x V2, few relevant works have been reported in the literature (to date) which focuses exclusively on enhancing the application potential windows of their prototype supercapacitor cells. These relevant literatures are briefly highlighted below:
[003] Tetraethylammonium tetrafluoroborate [Et4N][BF4]-organic conducting salt dissolved in polar acetonitrile (ACN) solvent is the most extensively utilized electrolyte for commercial supercapacitor cells. This electrolyte has lesser viscosity along with decent ionic conductivity under ambient conditions. But the primary concern is its marginally lower operating potential span (2.5 to 2.7 V) and therefore, it exhibits a lower energy density. Scientists have devoted their extreme efforts in developing miscellaneous pertinent electrolytes for supercapacitor cells to augment the overall energy density. In this regard, reference is made to 2015, Schu¨tter et al. The Journal of Physical Chemistry C 119 (2015):13413-13424: https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.5b02113] reported (by utilizing both experimental as well as computational approaches) which reported the physicochemical performances of two different nitrile-based solvents (glutaronitrile and 2-methylglutaronitrile) along with [Et4N][BF4]. However, a major issue with such electrolytic solvents is their intrinsically weak ion transport properties.
[004] Further reference is made to Ding, M. S., et al. Journal of Power Sources 138 (2004): 340-350: https://www.sciencedirect.com/science/article/abs/pii/S0378775304006949], which reported a binary eutectic solvent mixture (?-butyrolactone + acetonitrile) along with an asymmetric-configured organic salt (triethylmethylammonium tetrafluoroborate) as an electrolyte for double-layer capacitors. Unfortunately, they have not reported the rate performance, energy, and power characteristics (especially under elevated scan rates/current densities).
[005] Further, room-temperature ionic liquids (ILs) are the newly established propitious participants in the electrolyte family for energy storage applications. These ILs exhibit several fascinating features/merits like wide-ranging applicable voltage windows, admirable electrochemical and thermal stability, tunable polarity, lower flammability, and negligible volatility [Armand, M., et al. Nature Publishing Group (2011):129-137: https://www.worldscientific.com/doi/abs/10.1142/9789814317665_0020 and Van Aken, K. L., et al. Angewandte Chemie 127 (2015): 4888-4891: https://onlinelibrary.wiley.com/doi/abs/10.1002/ange.201412257]. Hence, introducing room-temperature ILs (as electrolytes) are known to improve the applicable working voltage range and subsequently the energy density.
[006] Reference is made to Kwon, H. N., et al. Scientific Reports 9 (2019):1-6:https://www.nature.com/articles/s41598-018-37322-y, in which the electrochemical performance of diverse ionic liquids (added as co-salt, separately) along with traditional [Et4N][BF4] organic salt was studied in propylene carbonate (as the electrolytes) for high voltage EDLCs. The authors in said article employed those ionic liquids individually to enhance the cell working potential window. It is noteworthy, the incorporation of propylene carbonate solvent in commercial supercapacitor cells is indeed very difficult owing to its greater viscosity along with inferior ionic conductivity. Besides, the authors of said article have not reported the rate performance, energy, and power characteristics (especially under elevated scan rates/current densities).
[007] Another reference is made to Tian, J., et al. Journal of Materials Chemistry A 6 (2018):3593-3601: https://pubs.rsc.org/en/content/articlelanding/2018/ta/c7ta10474j/unauth which employed an ionic liquid diluted with ?-butyrolactone solvent (as unique binary electrolyte system at a volumetric ratio of 1:1) for high-voltage (3.7 V) graphene-based EDL-type supercapacitor. But utilizing higher volume of expensive ionic liquids is not viable at the industrial level. Besides, they have not stated the rate performance, and power characteristics at elevated scan rates/current densities (under ambient conditions).
[008] One another reference is made to Li, X., et al. Frontiers in Materials 8 (2021):1-8: https://www.frontiersin.org/articles/10.3389/fmats.2021.633460/full, who formulated a complex electrolyte by combining 1.5 M ionic liquid with 0.1 M lithium hexafluorophosphate (LiPF6) dissolved in a mixed carbonate solvent (propylene carbonate:dimethyl carbonate = 1:1). However, it was observed that the cost of preparing such electrolyte (with higher ionic liquid content) on industrial scale was considerably higher which simultaneously exaggerated the watt-hour cost of cells. Owing to immense rise in the overall production cost (watt-hour cost) of such high voltage electrolytes (particularly), their future industrial viability is minuscule.
[009] Thus, the literature describes the functionalities of various unique high-voltage electrolytes (using suitable solvents and/or co-salts) for supercapacitor purposes.
[010] Despite diverse advantages, there are several drawbacks of these room-temperature ILs which eventually restrict their industrial utility/viability. Their substantially high costs, purity concerns, higher viscosity/? (as compared to the commercially available organic electrolytes), relatively lower ionic conductivity (s) issues and poor contact with the participating electroactive materials (particularly with carbonaceous-based) engender massive obstacles to scale-up their productions for the pilot-line cells manufacturing [Pan, S., et al. Frontiers in Chemistry 8, No.: 261 (2020):1-18: https://www.frontiersin.org/articles/10.3389/fchem.2020.00261/full and Kwon, H. N., et al. Scientific Reports 9 (2019):1-6: https://www.nature.com/articles/s41598-018-37322-y]. Their high ? and relatively poor s values (when used as pristine/pure IL-electrolyte) essentially reduces the overall cell power density/P with a subsequent increment of the corresponding cell internal resistance. All those above-mentioned shortcomings of room-temperature ILs need to be overcome for frequent industrial utility as electrolytes in diverse energy storage devices.
[011] Accordingly, there is a dire need in the art to provide a high voltage electric double layer capacitor supercapacitor cell (comprising high surface area, porous, activated carbon-electrodes) which would be effective enough to improve the overall energy density.

OBJECTIVES OF THE INVENTION:
[012] The principal object of the present invention is to provide a high voltage electric double layer capacitor (EDLC).
[013] Another object of the present invention is to provide an electrolyte comprises an electrolyte salt and a combination of solvents.
SUMMARY OF THE INVENTION:
[014] The present invention discloses a high voltage electric double layer capacitor (EDLC), comprising: (I) one or more cathode, wherein said cathode is fabricated by following steps: (a) mixing a cathode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final cathode slurry; and (c) coating the final cathode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more cathode; (II) one or more anode, wherein said anode is fabricated by following steps: (a) mixing an anode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final anode slurry; and (c) coating the final anode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more anode; and (III) an electrolyte. The present invention also discloses an electrolyte comprising an electrolyte salt and a combination of solvents, and wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent. The high voltage electric double layer capacitor (EDLC) comprising the electrolyte as described herein exhibits a higher energy density, a wide electrochemical stable potential window (compared to commercial EDLCs), and moderate power characteristics.

DESCRIPTION OF ACCOMPANYING FIGURES:
[015] The accompanying drawings constitute a part of the description and are used to provide further understanding of the present invention. Such accompanying drawings illustrate the embodiments of the present invention, which are used to describe the principles of the present invention together with the description.
[016] Figure 1 depicts the process followed for our high voltage (3.2 V) EDLC electrolyte formulation and corresponding cells assembly in a stepwise manner, in accordance with an implementation of the present invention.
[017] Figure 2 depicts the processes followed for high-voltage (3.2 V) EDLC electrolyte formulations. Four different types of EDLC electrolytes were formulated for comparison studies, in accordance with an implementation of the present invention.
[018] Figure 3 depicts the stepwise or stagewise protocols for preparing the common polymeric binder stock solution (as shown in Stage-1), preparation of the positive electrode (with proper mass loading acceptable for industrial pilot-scale; as shown in Stage-2), and preparation of the negative electrode (with proper mass loading acceptable for industrial pilot-scale; as shown in Stage-3), concisely, at dry-room condition RH = 10%. Stage-1 illustrates the standard preparation process of the common polymeric binder stock solution, in accordance with an implementation of the present invention.
[019] Figure 4a depicts the slurry viscosity study of the negative electrode consists of activated carbon-1900 (as the active material), conductive carbon additives (with two different surface areas), and binder (fluoride-based) with a minimum 5% (weight) in pyrrolidone-based solvent-1, in accordance with an implementation of the present invention.
[020] Figure 4b depicts the slurry viscosity study of the positive electrode (prepared by using the same protocols and weight ratio as-like anode) comprised of activated carbon-3000 (as the active material), conductive carbon additives (with two different surface areas), and binder (fluoride-based) with a minimum 5% (weight) in pyrrolidone-based solvent-1, in accordance with an implementation of the present invention.
[021] Figure 4c depicts the modified slurry viscosity study of the positive electrode (prepared by employing slightly different protocols and weight ratios) comprised of activated carbon-3000 (as the active material), conductive carbon additives (with two diverse surface areas), and binder (fluoride-based) at with a minimum 7% (weight)in pyrrolidone-based solvent-1, in accordance with an implementation of the present invention.
[022] Figure 5a depicts the as-acquired galvanostatic charge-discharge curves of four fabricated EDLC cells with different configurations, in accordance with an implementation of the present invention.
[023] Figure 5b depicts a brief comparative study of the acquired energy density of the three corresponding cells (cell-2, cell-3 and cell-4) within a potential window of 0-3.2 V, in accordance with an implementation of the present invention.
[024] Figure 6a depicts cyclic voltammetry (CV) profiles of cell-3 and cell-4 within 0-3.2V potential window, in accordance with an implementation of the present invention.
[025] Figure 6b depicts an enlarged view of Figure 6a-within 2.6 V-3.2 V potential window, in accordance with an implementation of the present invention.
[026] Figure 7a depicts the acquired galvanostatic charge-discharge curves of final cell-4 at different current densities, in accordance with an implementation of the present invention.
[027] Figure 7b depicts the comparative Galvanostatic Cycling with Potential Limitation (GCPL) cyclic stability plots of three as-fabricated cells (Cell-1, Cell-3, and Cell-4) cycled at 1 A/g constant current density within 0-3.2 V for 3000 continuous cycles, in accordance with an implementation of the present invention.
DETAILED DESCRIPTION OF THE INVENTION:
[028] While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the scope of the invention as defined by the appended claims.
[029] 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.
[030] The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
[031] As used herein, the term “high voltage electric double layer capacitor” refers to an electric energy storage system based on charge–discharge process (electrosorption) in an electric double layer on porous electrodes, which are used as memory back-up devices because of their high cycle efficiencies and their long life-cycles.
[032] As discussed in the background section of the present invention, there exist many challenges/shortcomings associated with the conventional electrolytes and capacitor cells comprising said conventional electrolytes. The challenges/shortcomings are poor rate performances, lower ionic conductivity associated with higher viscosity, inferior power density, and stability which need to be overcome for using such pertinent high-voltage electrolytes in commercial supercapacitor cells (industrial level).
[033] To overcome the aforementioned problems associated with supercapacitor power-cells fabrication such as higher electrolyte ?, higher internal resistances, lower s, inferior rate capability, stability issues at high voltages (electrolyte decomposing), parasitic side reactions, and poor contact with the electroactive materials, the present invention provides an electrolyte formulation and high voltage electric double layer capacitor that exhibits an improved s and electrochemical stability, comparatively lower ?, decent rate capability, a wide electrochemical stable voltage window associated with considerably higher energy density (compared to conventional electric double layer/EDL supercapacitors), decent power characteristics, adequate wettability, and virtually negligible parasitic side reactions.
[034] To be specific, the present invention provides the formulations/preparation of a typical high voltage, an organic-based liquid electrolyte for commercial supercapacitor power cells (consisting of porous high surface area carbonaceous electrodes). To demonstrate the industrial viabilities, the formulated electrolyte comprising an electrolyte salt and a combination of solvents (Gamma-Butyrolactone and Propylene carbonate), was used to fabricate the cylindrical supercapacitor cell comprising of two calendared carbonaceous electrodes separated by a cellulose-based paper separator (25- 40 µm thick). The formulations and optimizations stages were accomplished preliminary on coin cells. This commercially viable innovative electrolyte is easy to formulate/prepare with exciting attributes such as lower viscosity (as compared to the high voltage ionic-liquids or propylene carbonate-based systems), ample ionic conductivity, low volatility, superior rate performance, wide electrochemical stable potential window associated with impressive cyclic stability, higher energy density (as compared to the commercial EDLCs) and adequate power.
[035] Thus, the electrolyte of the present invention shows the following breathtaking features:
(i) An unconventional (asymmetric-structured) organic conducting- salt (tetrafluoroborate based) with adequate ionic conductivity is deployed as a supporting electrolyte salt. This salt helps directly to alleviate the concerns of lower ionic conductivity and poor ions-pore interactions, as compared to the traditionally utilized TEABF4 organic salt (dissolved in propylene carbonate or adiponitrile solvent) and pure ionic liquids-based high voltage organic electrolytes. This distinctive asymmetric-structured tetrafluoroborate-based salt exhibits better electrochemical performances and physicochemical properties. This is owing to the smaller cation size (as well as solvated ion size) of the employed asymmetric-structured tetrafluoroborate-based salt than the traditional TEA+ in identical solvents. Such a phenomenon enhances ionic mobility/ ionic conductivity as well.
(ii) A lactone-based aprotic polar solvent is deployed as the electrolyte solvent/medium for for-mulating/preparing our exclusive high voltage (= 3.4 V) supercapacitor electrolyte. It is to be stat-ed that this special lactone-based organic solvent was carefully chosen owing to its wide electro-chemical stable potential window and lesser propensity to degrade/decompose on the high sur-face area carbonaceous electrodes surface, even at a higher applied potential. It also reveals better ionic conductivity/mobility and lower viscosity as compared to propylene carbonate/adiponitrile. Even though, lactone-based electrolytes cannot compete with the high conductivity acetonitrile-based electrolytes. But evidently, they satisfy the global demand for high energy density at mod-erate power. Two diverse solvents such as cyclic lactone-based (as the main solvent) e.g., Gam-ma-Butyrolactone (GBL) along with a cyclic carbonate-based co-solvent such as Propylene car-bonate (PC) were used. While standard/commercial EDLC cells generally employ solely nitrile-based e.g., Acetonitrile (ACN) or carbonate-based electrolytes e.g., PC, which have very high ionic conductivity but unfortunately, they can provide a maximum voltage window of 2.5-2.7 V. Whereas, carbonate-based electrolytes although can withstand a potential window of 3.0 V but they have very high viscosity associated with a lower ionic conductivity which eventually reduc-es the overall cell performances. Henceforth, lactone-based solvent along with carbonate-based co-solvent were used to formulate a high voltage 3.2 V EDLC electrolyte.

[036] It is also noteworthy to note that only formulating and incorporating electrolyte of the present invention doesn’t successfully produce high voltage EDLC cells. It is equally important to properly optimize the design and protocols of fabricating associated positive and negative electrodes. Accordingly, the present invention also provides a high voltage EDLC cell with dif-ferent mass ratios (cathode: anode) in order to balance the charges generated both on the cathode and anode-sides during electrochemical cycling. Increasing the energy density of a particular cell is a very essential parameter for the modern energy storage research in order to install their cells for commercial appliances (such as electric vehicles and grids). Therefore, to increase the energy density, the voltage window was also increased along with the specific capacity of the final cell. Further, in order to mitigate the concern of higher energy density, a very high surface area acti-vated carbon (specific surface area 28003200 m2/g) was employed as cathode and moderate-to-high surface area activated carbon (specific surface area 1600-1900 m2/g) was employed as an-ode. In standard EDLC coin cells, the material-based specific capacitance is approximately 80-90 F/g and the delivered energy density is around 22 Wh/kg (materials-level). The high voltage EDLC cells (with two diverse surface area active carbon separated by as-formulated electrolyte) exhibited material-based specific capacitance of approximately 116.2 F/g and the delivered ener-gy density of around 39.8 Wh/kg (materials-level).

[037] Thus, the above results in an outcome of the synergistic contributions of gamma-Butyrolactone, propylene carbonate, and Triethylmethylammonium Tetrafluoroborate as an elec-trolyte salt that are used for preparing the electrolyte of the present invention which is used for fabricating the high voltage EDLC cells. The high voltage EDLC cells comprises a very high sur-face area activated carbon (specific surface area in the range of 2800 m2/g to 3200 m2/g) as a cathode and moderate-to-high surface area activated carbon (specific surface area in the range of 1600 m2/g to 1900 m2/g) as an anode and that the high voltage EDLC exhibits a higher energy density, a wide electrochemical stable potential window (compared to commercial EDLCs), and moderate power characteristics.

[038] In view of the above, the major differences between the present invention and the existing technology are as follows:
(1) Generally, the conventional EDLC-supercapacitor power cells comprise of carbonaceous electroactive materials (with very high surface area 1000-2200 m2/g) and TEABF4 (dissolved in ACN) as the electrolyte. These cells demonstrate excellent power densities, cyclic stability, rate capabilities, and low internal resistance. But the major concern of all such supercapacitor cells is their low applicable voltage windows (= 2.7-2.8 V) and inadequate energy density. In order to mitigate such issues, the present invention provides the formulation of a typical high voltage commercially viable supercapacitor electrolyte by judiciously incorporating a distinct tetrafluoroborate-based organic conducting salt, and a combination of polar aprotic lactone-based solvents. The formulated electrolyte of the present invention exhibits a remarkably higher energy density, a wide electrochemical stable voltage window (compared to traditional EDLCs), and moderate power characteristics. According to the literature, few newly developed high-voltage EDLC electrolytes possess propylene carbonate (PC) as the solvent with TEABF4. Although, PC solvent can withstand up to a plentiful higher potential, the presence of high viscous PC solvent (with inadequate ionic conductivity) reduces the overall power characteristics and rate capabilities of the associated supercapacitor cells. Likewise, it enhances the cell’s internal resistance. Therefore, PC-based electrolytes are not commercially viable/scalable at all. The present invention provides a typical lactone-based aprotic polar solvent which exhibited a significantly lower viscosity and higher ionic mobility than PC-based electrolytes. It also shows low volatility, low internal resistance and ample power characteristics.
(2) Commonly, the commercial EDLC supercapacitors possess symmetric-structured organic conducting salt i.e., TEABF4. However, in the present invention, a typical asymmetrically structured tetrafluoroborate-based organic conducting salt was employed. It shows higher ionic conductivity and ample ion mobility (compared to the conventional TEABF4 salt). Besides, it exhibits a lower degree of solvation in aprotic solvents.
(3) According to the literature, incorporation of solitary ionic liquids is obligatory for preparing high-voltage supercapacitor electrolytes. But the major obstacle is its high cost and lower purity. Thus, the utility of ionic liquids at scalable/commercial level is not viable at all. In light of these limitations, a trivial extent of imidazolium-based ionic liquid was used as a specific electrolyte additive (available widely) to solve the objective. The addition of the same eventually enhances the electrochemical stable potential window, overall ionic conductivity, ion density, and room temperature ion transport properties of the formulated electrolyte of the present invention.
[039] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC), comprising: (I) one or more cathode, wherein said cathode is fabricated by following steps: (a) mixing a cathode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final cathode slurry; and (c) coating the final cathode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more cathode; (II) one or more anode, wherein said anode is fabricated by following steps: (a) mixing an anode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final anode slurry; and (c) coating the final anode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more anode; and (III) an electrolyte.

[040] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the cathode active material is an activated Carbon-3000 and the anode active material is an activated Carbon-1900.

[041] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the conductive carbon additive of cathode has a specific surface area in the range of 2800 m2/g to 3200 m2/g and the conductive carbon additive of cathode has a specific surface area in the range of 1600 m2/g to 1900 m2/g. In another embodiment of the present invention, the conductive carbon additive of cathode has a specific surface area in the range of 2900 m2/g to 3100 m2/g and the conductive carbon additive of cathode has a specific surface area in the range of 1700 m2/g to 1900 m2/g.

[042] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the active material: conductive additive: binder of cathode has a weight ratio of 88:5:7 and the active material: conductive additive: binder of anode has a weight ratio of 95:5:5.

[043] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the cathode has a binder at a weight percentage in the range of 7-10% and the anode has a binder at a weight percentage in the range of 3-5%. In another embodiment of the present invention, the cathode has a binder at a weight percentage in the range of 7-9%, or 7-8% and the anode has a binder at a weight percentage in the range of 3-5%, or 3-4%.

[044] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the particle size of final cathode slurry and final anode slurry is in the range of 15-20 microns. In another embodiment of the present invention, the particle size of final cathode slurry and final anode slurry is in the range of 16-19 microns, or 17-18 microns.

[045] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the final cathode slurry has a viscosity of 3318.2 mPa.s@10 Sheer Rate and the final anode slurry has a viscosity of 2351.5 mPa.s@10 Sheer Rate.

[046] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the electrolyte has a voltage of 3.2 V.

[047] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the high voltage electric double layer capacitor exhibits a specific capacitance of 116.2 F/g and a delivered energy density of 39.8 Wh/kg.

[048] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the vacuum drying temperature is 120°C.

[049] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the electrolyte comprises an electrolyte salt and a combination of solvents.

[050] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the electrolyte comprises an electrolyte salt and a combination of solvents, and wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent.
[051] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the electrolyte comprises an electrolyte salt and a combination of solvents, and wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent, and wherein the main solvent is a lactone based solvent and the co-solvent is a carbonate based solvent, and wherein the main solvent is Gamma-Butyrolactone and the co-solvent is Propylene carbonate.

[052] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the electrolyte salt is Triethylmethylammonium Tetrafluoroborate.

[053] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the cathode active material is an activated Carbon-3000 and the anode active material is an activated Carbon-1900, and wherein the conductive carbon additive of cathode has a specific surface area in the range of 2800 m2/g to 3200 m2/g and the conductive carbon additive of cathode has a specific surface area in the range of 1600 m2/g to 1900 m2/g.

[054] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the active material: conductive additive: binder of cathode has a weight ratio of 88:5:7 and the active material: conductive additive: binder of anode has a weight ratio of 95:5:5, and wherein the final cathode slurry and final anode slurry has a particle size is in the range of 15-20 microns. In another embodiment of the present invention, the final cathode slurry and final anode slurry has a particle size is in the range of 16-19 microns, or 17-19 microns, or 18-19 microns.

[055] In an embodiment of the present invention, there is provided a high voltage electric dou-ble layer capacitor (EDLC) as described herein, wherein the final cathode slurry has a viscosity of 3318.2 mPa.s@10 Sheer Rate and the final anode slurry has a viscosity of 2351.5 mPa.s@10 Sheer Rate, and wherein the electrolyte has a voltage of 3.2 V, and wherein the high voltage electric double layer capacitor exhibits a specific capacitance of 116.2 F/g and a delivered energy density of 39.8 Wh/kg.

[056] In an embodiment of the present invention, there is provided a high voltage electric double layer capacitor (EDLC) as described herein, wherein the cathode active material is an activated Carbon-3000 and the anode active material is an activated Carbon-1900, and wherein the conductive carbon additive of cathode has a specific surface area in the range of 2800 m2/g to 3200 m2/g and the conductive carbon additive of cathode has a specific surface area in the range of 1600 m2/g to 1900 m2/g, and wherein the active material: conductive additive: binder of cathode has a weight ratio of 88:5:7 and the active material: conductive additive: binder of anode has a weight ratio of 95:5:5, and wherein the final cathode slurry and final anode slurry has a particle size is in the range of 15-20 microns, and wherein the final cathode slurry has a viscosity of 3318.2 mPa.s@10 Sheer Rate and the final anode slurry has a viscosity of 2351.5 mPa.s@10 Sheer Rate, and wherein the electrolyte has a voltage of 3.2 V, and wherein the high voltage electric double layer capacitor exhibits a specific capacitance of 116.2 F/g and a delivered energy density of 39.8 Wh/kg, and wherein the vacuum drying temperature is 120°C, and wherein the electrolyte comprises an electrolyte salt and a combination of solvents, and wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent, and wherein the main solvent is a lactone based solvent and the co-solvent is a carbonate based solvent, and wherein the main solvent is Gamma-Butyrolactone and the co-solvent is Propylene carbonate, and wherein the electrolyte salt is Triethylmethylammonium Tetrafluoroborate.

[057] The present invention is illustrated hereunder in greater detail in relation to non-limiting exemplary embodiments as per the following examples.

[058] The forthcoming examples discusses in detail the objective of the present invention that lies in:- (a) Designing of wider potential electrolyte with low viscosity; (b) Optimizing the volume concentration of high viscosity and low viscosity solvent to design the electrolyte; (c) Selection of appropriate electrolyte salt (huge difference in cationic and anionic ionic radius); (d) Charge-balance of the positive and negative electrode by utilizing two different surface area activated carbon. (Positive electrode: =3000 m2/g and negative electrode: =1900 m2/g) and (e) Slurry viscosity optimization of high surface area electrode to achieve the optimized mass loading to design the positive electrode.
EXAMPLES
[059] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and the description of how to make and use the present invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all and only experiments performed. The methodology of preparing few of the preferred embodiments shall become clearer with working examples provided below.
Example 1: Electrolyte formulation and process for preparing the electrolyte formulation:
[060] The present invention provides an electrolyte formulation comprising 1.2 M fluoroborate-based (asymmetric structured) organic conducting salt and combination of solvents, i.e., 70% of ? butyle lactone (GBL) (cyclic lactone-based solvent) (main solvent) and 30% of propylene carbonate (a cyclic carbonate-based co-solvent).
[061] The electrolyte formulation of the present invention was prepared by following the steps as depicted in Figure 1. The high voltage electrolyte (3.2 V) was formulated inside an Ar-filled glove-box (H20 = 0.05 PPM and 02 = 0.05 PPM) by employing 1.2 M fluoroborate-based (asymmetric structured) organic conducting salt dissolved in homogeneous mixture of cyclic lactone-based solvent, i.e., ? butyrolactone (GBL) (70%) (99% purity) along with a cyclic carbonate-based co-solvent, i.e., propylene carbonate (PCL) (30%) (99.6% purity) (Figure 2). It is noteworthy that the organic conducting salt was dried at 120 °C under vacuum (in-side glove-box antechamber) for 24 hrs, while the associated solvents were dried by employing molecular sieves prior to formulating the final electrolyte.

Comparative electrolyte formulations
[062] Four different types of electric double layer capacitor (EDLC) electrolytes were formulated for comparison studies (Figure 2). The process of all diverse-types electrolytes (Electrolyte-1, 2, 3, and Standard Electrolyte-4) formulations was identical. Unique Electrolyte-1 (working electrolyte of the present invention) was prepared by merely dissolving a high purity (asymmetric structured) fluoroborate-based highly conducting organic salt in a judicious mixture of lactone-based (cyclic-structured; Solvent-1) and carbonate-based (cyclic-structured; Solvent-2) solvents by following the process as described above. It is noteworthy, the preparations of all organic electrolytes were accomplished inside an Ar-filled glove box with H2O content = 0.05 PPM and O2 content = 0.05 PPM using a standard table-top magnetic stirrer. Likewise, Electrolyte-2, Electrolyte-3 and Standard Electrolyte-4 were prepared using the same protocols with solely lactone-based (Solvent-1) such as ?-butyrolactone, carbonate-based (Solvent-2) such as propylene carbonate, and traditional nitrile-based (linear-structed) solvents such as acetonitrile (ACN), consequently, for comparative electrochemical analyses.
[063] Finally, the corresponding EDLC cells were assembled with the aforementioned electrolytes. Upon performing comparative electrochemical analyses, it was observed that standard/commercial EDLC cells generally employ solely nitrile-based e.g., ACN or carbonate-based electrolytes e.g., PC. Nitrile-based electrolytes (Electrolyte 4) have very high ionic conductivity but unfortunately, they provided a maximum voltage window of 2.5-2.7 V. Whereas, carbonate-based electrolytes (Electrolyte 3) although withstood a potential window of 3.0 V but they had a very high viscosity associated with a lower ionic conductivity which eventually reduced the overall cell performances. Henceforth, the working electrolyte (working electrolyte 1) comprising lactone-based solvent along with carbonate-based co-solvent withstood a high voltage of 3.2 V.
Example 2: High voltage electric double layer capacitor (EDLC) cells and process for preparing thereof
[064] In the present example, EDLC cells was assembled with the working electrolyte (working electrolyte 1) comprising 1.2 M fluoroborate-based organic conducting salt and a homogeneous mixture of cyclic lactone-based solvent, i.e., ? butyrolactone (GBL) (70%) along with a cyclic carbonate-based co-solvent, i.e., propylene carbonate (PCL) (30%). The ultimate high operating voltage electrical double layer capacitor cell was assembled by employing positive electrode (surface area in the range of 2800 m2/g to 3200 m2/g) and negative electrode (surface area in the range of 1600 m2/g to 1900 m2/g), respectively.

[065] Figure-3 demonstrates the stepwise or stage wise protocols for preparing the common polymeric binder stock solution (as shown in Stage-1), preparation of the positive electrode (with proper mass loading acceptable for industrial pilot-scale; as shown in Stage-2), and preparation of the negative electrode (with proper mass loading acceptable for industrial pilot-scale; as shown in Stage-3), concisely, at dry-room condition RH = 10%.

[066] Stage-1 illustrates the standard preparation process of the common polymeric binder stock solution. Primarily, in stage 1, a fluoride-based polymer powder (pre-dried at 80°C for 24 h under vacuum) with an average molecular weight of 500 x 103 g/mole was dissolved in a suitable pyrrolidone-based colorless organic solvent under constant stirring condition overnight at around 500-700 RPM. After overnight stirring, a transparent and viscous polymeric binder stock solution was collected, which was later commonly used for both cathode (higher quantity) and anode (lower quantity) fabrications.
[067] Stage-2 and Stage-3 demonstrates the preparation of the positive and negative electrodes, separately.
[068] For the preparation of the positive electrode (Stage-2), first activated carbon-3000 (surface area in the range of 2800 m2/g to 3200 m2/g) was dry-mixed along with conductive carbon additives to achieve a consistently mixed powder which was further mixed with the formerly prepared polymeric binder solution (in measured quantity) and additional pyrrolidone-based solvent-1 to acquire a wet mixed slurry composition. Afterwards, through slurry mixing (via essential viscosity optimization steps for a high areal cathode mass loading and particle size measurement) was carried out to get the final cathode slurry. During this stage, solid: liquid ratio was maintained as 1:3.9 and binder to liquid ratio was maintained as 1:56 to achieve consistent/uniform cathode slurry. Finally, the positive electrode slurry was gently coated on an aluminum-foil current collector and subsequently was vacuum dried at 120°C for 24 h to develop cathode component electrode. During the process, no peel-out was observed in the final cathode. For the preparation of anode (Stage-3), at first activated carbon-1900 (surface area in the range between 1600 m2/g to 1900 m2/g) was dry-mixed along with conductive carbon additives to achieve a consistently mixed powder which was further mixed with the formerly prepared polymeric binder solution (in measured quantity) and additional pyrrolidone-based solvent-1 to acquire a wet mixed slurry composition. Afterwards, thorough slurry mixing (via essential viscosity optimization steps for a lower areal anode mass loading and particle size measurement) was carried out to get the final anode slurry. Here, solid: liquid ratio was maintained as 1:3 and binder: liquid ratio was maintained as 1:62 to achieve uniform anode slurry. Finally, this anode slurry was gently coated on aluminum-foil current collector and subsequently vacuum dried at 120°C for 24 h to develop anode component electrode. During this process also, no peel-out was observed in the final anode.

Example 3: Slurry viscosity study of the positive electrode and negative electrode

[069] It is to be noted that only formulating and incorporating electrolytes (such as working electrolyte 1 of the present invention) is not only feature that is essential for producing high voltage EDLC cells. In addition to the electrolyte deployed in the present invention, different design and protocols of fabricating associated positive and negative electrodes are equally important. In the present invention, the EDLC cell was designed with different mass ratios (cathode: anode) in order to balance the charges generated both on the cathode and anode-sides during electrochemical cycling.

[070] Increasing the energy density of a particular cell is a very essential parameter for the modern energy storage research in order to install their cells for commercial appliances (such as electric vehicles and grids). Therefore, to increase the energy density, it is important to increase the voltage window along with the specific capacity of the final cell. Therefore, in order to mitigate the concern of higher energy density, a very high surface area activated carbon (specific surface area ˜ 3000 m2/g) was employed as cathode and moderate-to-high surface area activated carbon (specific surface area ˜ 1900 m2/g) was employed as anode.

[071] Figure 4a demonstrates the slurry viscosity study of the negative electrode consisting of activated carbon-1900 (as the active material), conductive carbon additives (with two different surface areas), and binder (fluoride-based) at a weight ratio of 95:5:5 in pyrrolidone-based solvent-1. The slurry viscosity was measured at 10 sheer rate (to get a constant estimated value without much variation in magnitude). The measured slurry viscosity for anode was 2351.5 mPa.s and the particle size of the uniform/consistent slurry was 15-20 microns.

[072] Figure 4b demonstrates the slurry viscosity study of the positive electrode (prepared by using the same protocols and weight ratio as-like anode) comprising of activated carbon-3000 (as the active material), conductive carbon additives (with two different surface areas), and binder (fluoride-based) at a weight ratio of 95:5:5 (same as anode slurry) in pyrrolidone-based solvent-1. The measured slurry viscosity for cathode was 2969.8 mPa.s@10 sheer rate and the particle size of the uniform/consistent slurry was bit higher 30-35 microns. After coating with this slurry, some issues appeared such as less viscosity to achieve higher mass loading, non-uniform coating (thickness variations), peel-out issues, and large particles/agglomerates in the dried electrodes. Henceforth, the slurry parameters were further modified (as revealed in Figure 4c) to achieve proper/ consistent slurry suitable for cathode coating.

[073] Figure-4c demonstrates the modified slurry viscosity study of the positive electrode (prepared by employing slightly different protocols and weight ratios) comprised of activated carbon-3000 (as the active material), conductive carbon additives (with two diverse surface areas), and binder (fluoride-based) at a weight ratio of 88:5:7 in pyrrolidone-based solvent-1. The measured slurry viscosity for modified cathode was 3318.2 mPa.s@10 sheer rate and the particle size of the uniform/consistent slurry was 15-20 microns. This time issues were observed (like previous observations as in the case of Figure 4b) and the coating was uniform, scalable for pilot-level.

[074] Thus, it can be deduced from the Figure 4c that the weight ratio of active material, conductive carbon additives (with two diverse surface areas), and binder (fluoride-based) at a weight ratio of 88:5:7 in pyrrolidone-based solvent-1 is essential for fabricating the anode that has a uniform coating.

Example 4: Effect of varying the solvent used in the electrolyte deployed for assembling the EDLC cells.

[075] The present example demonstrates the effect of varying the solvent used for preparing the electrolyte that is used for fabricating the EDLC cells. Figure 5a reveals the as-acquired galvanostatic charge-discharge curves of four fabricated EDLC cells with different configurations. Cell-1 was fabricated by employing two identical activated carbon electrodes as cathode and anode (symmetrical cell configurations) immersed within the traditional/commercial nitrile-based electrolyte. As observed, cell-1 (with commercial/traditional nitrile-based electrolyte system) could not withstand up to 3.2 V potential (black dotted curve). The cell-2 was assembled by employing two identical activated carbon-electrodes as cathode and anode (symmetrical cell configuration) immersed within the pure lactone-based high voltage electrolyte. As observed, cell-2 (with lactone-based electrolyte) withstood up to 3.2 V exhibiting a gravimetric specific capacitance of 63 F/g (blue curve). Cell-3 was assembled by using two identical activated-carbon electrodes as cathode and anode (symmetrical cell configuration) immersed within the as-prepared high voltage electrolyte (fluoroborate-based organic conducting salt dissolved in a judicious mixture of lactone- and carbonate-based solvents) of the present invention. As observed, cell-3 (with as-prepared high voltage EDLC electrolyte, i.e., electrolyte 1) withstood up to 3.2 V exhibiting a substantially higher gravimetric specific capacitance of ~88.9 F/g (green dotted curve). The cell-4 was assembled by using two diverse activated carbon electrodes i.e., activated carbon-3000-as cathode and activated carbon-1900-as anode (asymmetrical cell configurations) immersed within as-prepared high- voltage electrolyte (fluoroborate-based organic conducting salt dissolved in a judicious mixture of lactone- and carbonate-based solvents; as in the case of electrolyte 1). Surprisingly, the cell-4 (with two diverse/asymmetric-type activated carbon electrodes and as-prepared high voltage EDLC electrolyte) withstood up to 3.2 V exhibiting a superior gravimetric specific capacitance of 116.2 F/g (brown curve).
Example 5: Comparative study of different EDLC cells.
[1] Figure-5b depicts a brief comparative study of the acquired energy density of the three cor-responding cells (cell-2, cell-3 and cell-4) within a potential window of 0-3.2 V. The cell-2 exhib-ited an energy density of 24 W h/kg while cell-3 exhibited an energy density of 31.3 W h/kg. Fi-nal cell-4 delivered the highest energy density of 39.8 W h/kg. It is to be stated that the as-calculated energy density was in device level. Thus, it is evidenced that the designed cell showed an increment of around 65% in energy density as compared to the conventional design (Figure 5b).
[076] Figure-6a reveals cyclic voltammetry (CV) profiles of cell-3 and cell-4 within 0-3.2V potential window. Figure 6b demonstrates an enlarged view of Figure 6a-within 2.6 V-3.2 V potential window. Referring to Figure 6a, it was observed that the area under the CV curve of cell-4 is substantially higher than cell-3. Furthermore, cell-3 had lower current response as compared to cell-4 which clearly reflects higher capacitance of the as-designed cell-4. Besides, cell-3 showed resistive behavior at higher voltage (2.6 to 3.2V) as shown in Figure 6b. This figure illustrates the CV performance of both the cells at high voltage regions.
[077] Figure-7a reveals the acquired galvanostatic charge-discharge curves of final cell-4 at different current densities. The high voltage EDLC cell-4 showed triangular shaped galvanostatic charge-discharge curves even at higher current densities. The final cell-4 was cycled repetitively within 0-3.2 V potential window even at an immensely higher current density. Figure 7b illustrates the comparative GCPL cyclic stability plots of three as-fabricated cells (Cell-1, Cell-3, and Cell-4) cycled at 1 A/g constant current density within 0-3.2 V for 3000 continuous cycles. Cell-1 (with fluoroborate-based organic conducting salt containing nitrile-based traditional EDLC electrolyte with symmetric electrode configuration) exhibited ~ 34.5 % of its initial specific capacitance retentions after 3000 cycles. In comparison, cell-3 showed 76.5 % of its initial gravimetric specific capacitance retentions after 3000 cycles. Finally, cell-4 showed around 94.4 % of its initial gravimetric specific capacitance retentions after 3000 cycles.
[078] Hence, it can be deduced that the designed high operating voltage electric double layer capacitor (EDLC) (as in the case of cell-4) comprising the asymmetric electrode configurations and high-voltage electrolyte provides extensively high energy density and substantial long-term cyclic stability.
[079] Overall, it can be inferred from the above results that in standard EDLC coin cells, the material-based specific capacitance is approximately 80-90 F/g and the delivered energy density is around 22 Wh/kg (materials-level). In the as-fabricated system (with two diverse surface area active carbon separated by as-formulated electrolyte) of the present invention, material-based specific capacitance was exhibited as approximately 116.2 F/g and the delivered energy density was around 39.8 Wh/kg (materials-level).

[080] Advantages of the present invention:

[081] The present invention discloses a high voltage electric double layer capacitor (EDLC) comprising: (I) one or more cathode, wherein said cathode is fabricated by following steps: (a) mixing a cathode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final cathode slurry; and (c) coating the final cathode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more cathode; (II) one or more anode, wherein said anode is fabricated by following steps: (a) mixing an anode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture; (b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final anode slurry; and (c) coating the final anode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more anode; and (III) an electrolyte. The present invention also discloses an electrolyte comprising an electrolyte salt and a combination of solvents, and wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent.

[082] The advantages of the high voltage electric double layer capacitor (EDLC) and electrolyte comprised in the capacitor of the present invention are as follows:
(a) Higher energy density;
(b) A wider electrochemical stable potential window as compared to commercial EDLCs and a high voltage of 3.2V;
(c) Moderate power characteristics;
(d) Large cyclic stability;
(e) Specific capacitance of 116.2 F/g and a delivered energy density of 39.8 Wh/kg; and
(f) Cost effective.
, Claims:1. A high voltage electric double layer capacitor (EDLC), comprising:

I. one or more cathode, wherein said cathode is fabricated by following steps:
a) mixing a cathode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture;
b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final cathode slurry; and
c) coating the final cathode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more cathode;

II. one or more anode, wherein said anode is fabricated by following steps:
a) mixing an anode active material with a conductive carbon additive to form a dry mixed powder, followed by addition of a polymeric binder solution to form a wet mixture;
b) optimizing viscosity and particle size measurement of the wet mixture of step (a), to form a final anode slurry; and
c) coating the final anode slurry of step (b), on an aluminum current collector, followed by its vacuum drying at a temperature in the range of 110°C-130°C to form one or more anode; and

III. an electrolyte.

2. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the cathode active material is an activated Carbon-3000 and the anode active material is an activated Carbon-1900.

3. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the conductive carbon additive of cathode has a specific surface area in the range of 2800 m2/g to 3200 m2/g and the conductive carbon additive of anode has a specific surface area in the range of 1600 m2/g to 1900 m2/g.

4. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the cathode has a binder at a weight percentage in the range of 7-10% and the anode has a binder at a weight percentage in the range of 3-5%.

5. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the final cathode slurry and final anode slurry has a particle size is in the range of 15-20 microns.

6. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the final cathode slurry has a viscosity of 3318.2 mPa.s@10 Sheer Rate and the final an-ode slurry has a viscosity of 2351.5 mPa.s@10 Sheer Rate.

7. The high voltage electric double layer capacitor (EDLC) as claimed in any one of the claims 1 to 6, wherein the electrolyte has a voltage of 3.2 V.

8. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the high voltage electric double layer capacitor exhibits a specific capacitance of 116.2 F/g and a delivered energy density of 39.8 Wh/kg.

9. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the vacuum drying temperature is 120°C.

10. The high voltage electric double layer capacitor (EDLC) as claimed in claim 1, wherein the electrolyte comprises an electrolyte salt and a combination of solvents.

11. The high voltage electric double layer capacitor (EDLC) as claimed in claim 10, wherein the combination of solvents is a homogeneous mixture of a main solvent and a co-solvent.

12. The high voltage electric double layer capacitor (EDLC) as claimed in claim 10 or 11, wherein the main solvent is a lactone based solvent and the co-solvent is a carbonate based solvent.

13. The high voltage electric double layer capacitor (EDLC) as claimed in claim 11 or 12, wherein the main solvent is Gamma-Butyrolactone and the co-solvent is Propylene car-bonate.

14. The high voltage electric double layer capacitor (EDLC) as claimed in claim 10, wherein the electrolyte salt is Triethylmethylammonium Tetrafluoroborate.

15. An electrolyte comprising: (a) an electrolyte salt; and (b) combination of solvents, wherein said solvent is a homogeneous mixture of main solvent and co-solvent.

16. The electrolyte as claimed in claim 15, wherein the main solvent is a lactone based solvent and the co-solvent is a carbonate based solvent.

17. The electrolyte as claimed in claim 15 or 16, wherein the main solvent is Gamma-Butyrolactone, co-solvent is Propylene carbonate and the electrolyte salt is Triethylme-thylammonium Tetrafluoroborate.

18. The electrolyte as claimed in claim 15, wherein the electrolyte salt is Triethylme-thylammonium Tetrafluoroborate.