Polymer Electrolyte with inorganic and organic salts for Na-Ion Supercapacitor

Abstract

The optimized high ion conducting polymer electrolyte film containing copolymer PVdF-HFP, 10 wt.% inorganic salt {sodium bis(trifluoromethane sulfonyl)imide (NaTFSI)} and 70 wt.% organic salt {1-butyl-1-methypyrrolidinium bis(trifluoromethane sulfonyl) imide ([BMPYr] [TFSI])} have been used for the fabrication of Na-ion supercapacitor. The optimized composition of the electrolyte film possesses maximum room temperature (RT) ionic conductivity (∼0.87 mS/cm), excellent mechanical stability and large operating potential window (∼5.5 V). Using this film and activated carbon electrode (ACE (AC∼0.8 mg/cm²)), electrochemical double layer capacitor (EDLC)/Na-ion supercapacitor has been fabricated. The bulk resistance of this cell is found to be 22.9 Ω which is an evidence of good electrode-electrolyte contact. The cyclic voltammetric (CV) results of the EDLC-cell displays almost rectangular shape which demonstrates their capacitive behavior. The fabricated Na-ion supercapacitor has delivered specific capacities of ∼173 F/g, 151.91 F/g, 145.32 F/g and 122.33 F/g at different areal current densities (∼0.5, 0.8, 1.0 and 2.0 mA/cm², respectively) along with the coulombic efficiency ranging from 97.6% to 99.9% upto 4500 cycles at 1 mA/cm². The obtained value of the specific capacitance(s) of the EDLC cell from cyclic voltammetry is in good agreement with the value obtained from galvanostatic charge-discharge (GCD) measurements. Also, a nearly stable cycling performance has been obtained at 1 mA/cm² upto 2500 cycles and after that the value of the specific capacitance (C_SP/CD) decreases slightly upto 4500 cycles. This decrease in C_SP/CD value may be because of increased thickness of solid electrolyte interface (SEI) layer and its corresponding interfacial resistance. The maximum specific energy and power density at 0.5 mA/ cm² areal current density for first cycle are 31.52 Wh/Kg and ∼472.8 kW/Kg, respectively. On using ACE having AC∼1.6 mg/cm², the fabricated Na-ion EDLC-cell has given maximum value of specific capacitance as ∼72.6 F/g at 0.5 mA/cm² alongwith coulombic efficiency in the range of 96.5 % to 99.5 %. For the first cycle, the energy as well power density increase (∼40.31 Wh/Kg and ∼499.12 kW/Kg, respectively) and show stable cyclability upto 3000 cycles.