Zn2+ Pre-Intercalation Stabilizes the Tunnel Structure of MnO2 Nanowires and Enables Zinc-Ion Hybrid Supercapacitor of Battery-Level Energy Density.

Although there has been tremendous progress in exploring new configurations of zinc-ion hybrid supercapacitors (Zn-HSCs) recently, the much lower energy density, especially the much lower areal energy density compared with that of the rechargeable battery, is still the bottleneck, which is impeding their wide applications in wearable devices. Herein, the pre-intercalation of Zn2+ which gives rise to a highly stable tunnel structure of Znx MnO2 in nanowire form that are grown on flexible carbon cloth with a disruptively large mass loading of 12 mg cm-2 is reported. More interestingly, the Znx MnO2 nanowires of tunnel structure enable an ultrahigh areal energy density and power density, when they are employed as the cathode in Zn-HSCs. The achieved areal capacitance of up to 1745.8 mF cm-2 at 2 mA cm-2 , and the remarkable areal energy density of 969.9 µWh cm-2 are comparable favorably with those of Zn-ion batteries. When integrated into a quasi-solid-state device, they also endow outstanding mechanical flexibility. The truly battery-level Zn-HSCs are timely in filling up of the battery-supercapacitor gap, and promise applications in the new generation flexible and wearable devices.