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

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 angstrom. 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. An optimized Zn hybrid electrolyte assures dendrite-free, reversible zinc plating and stripping at capacity densities up to 50 mAh cm-2 and discharge depths of 90%. A Zn||activated-carbon ion capacitor with this electrolyte operates at 2.0 to 2.5 V and achieves a capacity retention of 91% after 5000 cycles, an energy density of 29.3 Wh kg-1 with a power density of 14250 W kg-1 at a current density of 15 A g-1.image

Automated summary

A newly developed electrolyte enables dendrite-free growth and efficient reaction in zinc ion hybrid capacitors, resulting in stable performance under different conditions.