Introducing Na2SO4 in aqueous ZnSO4 electrolyte realizes superior electrochemical performance in zinc-ion hybrid capacitor

Zinc-ion hybrid capacitors (ZIHCs) are gaining much attention for their high energy density; however, the charge storage in traditional ZnSO4 electrolyte is limited by its low ionic conductivity, resulting in low specific capacity, rate capability, and gravimetric energy/power density at high rates. Herein, we report a ZIHC with high charge storage, enhanced rate capability, and superior energy/power density by introducing Na2SO4 in the traditional ZnSO4 electrolyte. The addition of Na2SO4 contributes immensely to the enhancement of capacitive charge storage in the activated carbon cloth positive electrode at high rates and shields the zinc anode from dendrite formation, resulting in excellent electrochemical performance. Specifically, the ZIHC with ZnSO4/Na2SO4 electrolyte presents high electrical and ionic conductivity, high specific capacity (152 mAh g−1), improved rate capability, and gravimetric energy density (20 Wh kg−1 at 1692 W kg−1). In contrast, the ZIHC with traditional ZnSO4 electrolyte delivers a much lower energy density at high rate (4.5 Wh kg−1 at 50 mA cm−2). In-situ Raman, nuclear magnetic resonance, and ex-situ X-ray diffraction demonstrate the inclusion of proton adsorption/desorption in the charge storage process. The proton storage process plays a vital role in the formation/dissolution of Zn(OH)2 analogs, extension of the voltage window to 1.9 V without compromising the Coulombic efficiency and overall charge storage. This article highlights the importance of electrolyte modification for enhancing the electrochemical performances of ZIHCs.