A New Free-Standing Aqueous Zinc-Ion Capacitor Based on MnO₂-CNTs Cathode and MXene Anode

Restricted by their energy storage mechanism, current energy storage devices have certain drawbacks, such as low power density for batteries and low energy density for supercapacitors. Fortunately, the nearest ion capacitors, such as lithium-ion and sodium-ion capacitors containing battery-type and capacitor-type electrodes, may allow achieving both high energy and power densities. For the inspiration, a new zinc-ion capacitor (ZIC) has been designed and realized by assembling the free-standing manganese dioxide-carbon nanotubes (MnO₂-CNTs) battery-type cathode and MXene (Ti₃C₂T_x) capacitor-type anode in an aqueous electrolyte. The ZIC can avoid the insecurity issues that frequently occurred in lithium-ion and sodium-ion capacitors in organic electrolytes. As expected, the ZIC in an aqueous liquid electrolyte exhibits excellent electrochemical performance (based on the total weight of cathode and anode), such as a high specific capacitance of 115.1 F g^-1 (1 mV s^-1), high energy density of 98.6 Wh kg^-1 (77.5 W kg^-1), high power density of 2480.6 W kg^-1 (29.7 Wh kg^-1), and high capacitance retention of ~ 83.6% of its initial capacitance (15,000 cycles). Even in an aqueous gel electrolyte, the ZIC also exhibits excellent performance. This work provides an essential strategy for designing next-generation high-performance energy storage devices.