A low-concentration 2-step synthesized spiroquaternary ammonium salt electrolyte for commercially stable pouch-type supercapacitors

Currently, non-aqueous, stable supercapacitors manufactured for commercial use require an environmentally benign and economically viable electrolyte synthesis technique. Spiro-based salts dissolved in organic solvents are known to have smaller ion-size, which facilitate charge transport during storage at varying temperatures and preserve storage efficiency over time. Previously, the synthesis of spirobased salt syntheses via complex multistep procedures involving toxic chemical solvents and low yields have been reported. The current work proposes a modified water-based ion-exchange process coupled with an innovative purification step that results in improved yield. In addition, the synthesized spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF4) salt mixed in an optimized organic solvent blend produced a dilute (0.2 M) electrolyte with its ionic conductivity comparable to commercially recognized, higher concentrated ionic salt electrolytes (3 M). Using the dilute SBPBF4 electrolyte, the pouch-type (PoC-SBF4) devices were operated up to 3 V. An average rated capacitance of 300 F was demonstrated with up to 43 W h kg−1 specific energy and 1 kW kg−1 of maximum deliverable power. After 10,000 continuous charge-discharge cycles, the device retained 97 % of its capacitance at an applied current of 5 A. Notably, the extreme temperature shock tests at −70 °C also demonstrated ~60 % capacitance retention after subjecting the device to cycling at 5 A for 72 h (3 days) to simulate extreme weather conditions for potential integration with renewable energy, mobility, or deep space applications. The water-based ion exchange step and the use of dilute electrolytes reduces the overall cost of manufacturing supercapacitor when scaling up the production process. The results presented provides a critical evaluation of the criteria for developing robust, stable, and low-cost supercapacitors from laboratory to large-scale manufacturing. [Display omitted] •Dual-step synthesis with good yield performed in water-based ion exchange process•Ultra stable device performance with dilute concentrations of the conductive salt•~ 60 % capacitance retention at extreme low temperatures up to −70 °C•Average specific energy of 43 W h kg−1 and corresponding power of 1 kW kg−1•Pouch-type design enables module assembly for integration in renewable energy units.