Multivalent ion storage towards high-performance aqueous zinc-ion hybrid supercapacitors

Multivalent ion storage mechanism is applied to construct high-performance aqueous zinc-ion hybrid supercapacitors (ZHSs). The constructed MnO2 nanorods//activated carbon (AC) ZHSs with ZnSO4 aqueous electrolyte are significantly different from the common MnO2//AC asymmetric supercapacitors with Na2SO4 electrolyte in electrochemical behaviors and energy storage mechanism. The ZHSs show the maximum specific capacity of 54.1 mA h g−1 and maximum energy density of 34.8 W h kg−1 at the optimal mass ratio of AC to MnO2. The ZHSs are capable of being charged/discharged rapidly within only 2–17 s, delivering a large power of 3.3–13.0 kW kg−1. Reversible insertion/extraction of Zn2+ into/from MnO2 nanorods and ion adsorption/desorption on AC particle surface, as well as partially reversible formation/dissolution of Zn4(OH)6SO4·nH2O participates on both electrodes, are established as the energy storage mechanism of the ZHSs. Electrochemical performance of the ZHSs can be further optimized through electrolyte composition regulation. Addition of Mn2+ cations into ZnSO4 electrolyte leads to significantly enhanced energy density (58.6 W h kg−1) of the ZHSs, while anion replacement of SO42- by CF3SO3- suppresses manganese dissolution and Zn4(OH)6SO4·nH2O formation, thus resulting in good cycling stability with 93.4% capacity retention over 5000 charge/discharge cycles. Overall, the ZHSs based on multivalent ion storage are promising to be applied as high-performance, extremely safe and eco-friendly energy storage devices for wearable/portable electronics and hybrid electric vehicles.