Key to High Performance Ion Hybrid Capacitor: Weakly Solvated Zinc Cations

Abstract

Zinc ion hybrid capacitors suffer from lack of reversibility and dendrite formation. An electrolyte, based on a solution of a zinc salt in acetonitrile and tetramethylene sulfone, allows smooth zinc deposition with high coulombic efficiency in a Zn||stainless steel cell (99.6% for 2880 cycles at 1.0 mA cm−2, 1.0 mAh cm−2). A Zn||Zn cell operates stably for at least 7940 h at 1.0 mA cm−2 with an area capacity of 10 mAh cm−2, or 648 h at 90% depth of discharge and 1 mA cm−2, 9.0 mAh cm−2. Molecular dynamics simulations reveal the reason for the excellent reversibility: The zinc cation is only weakly solvated than in pure tetramethylene sulfone with the closest atoms at 3.3 to 3.8 Å. With this electrolyte, a zinc||activated‐carbon hybrid capacitor exhibits an operating voltage of 2.0 to 2.5 V, an energy‐density of 135 Wh kg−1 and a power‐density of 613 W kg−1 at 0.5 A g−1. At the very high current‐density of 15 A g−1, 29.3 Wh kg−1 and 14 250 W kg−1 are achieved with 81.2% capacity retention over 9000 cycles.