Effect of Organic Solvents on the Electrochemical Performance of Sodium-Ion Hybrid Capacitors

A comparative study on the ionic conductivity of carbonate- and ether-based electrolytes was performed, which showed that the ionic conductivity was strongly affected by the dielectric constant of the organic solvent. The relationship between the physicochemical properties of a liquid electrolyte and the cycling performance of sodium-ion hybrid capacitors (SICs) was assessed by comparing the electrochemical characteristics of SICs assembled with a negative activated carbon electrode and a positive Na3V2(PO4)(3) electrode. Cyclic voltammetry, leakage current measurements, and electrochemical impedance spectroscopy results demonstrated that the good cycling performance of SICs with carbonate-based electrolytes can be ascribed to their superior electrochemical stability, large amounts of free ions, and favorable electrochemical reaction kinetics. An SIC employing an electrolyte consisting of 1.0 M NaClO4 in ethylene carbonate/propylene carbonate exhibited a good rate capability and delivered a high initial discharge capacity of 98.7 mAh g(-1) with a capacity retention of 90.8% after 1000 cycles at a constant current density of 500 mA g(-1).