Achieving Insertion-Like Capacity at Ultrahigh Rate via Tunable Surface Pseudocapacitance.

The insertion/deinsertion mechanism enables plenty of charge-storage sites in the bulk phase to be accessible to intercalated ions, giving rise to at least one more order of magnitude higher energy density than the adsorption/desorption mechanism. However, the sluggish ion diffusion in the bulk phase leads to several orders of magnitude slower charge-transport kinetics. An ideal energy-storage device should possess high power density and large energy density simultaneously. Herein, surface-modified Fe O quantum dots anchored on graphene nanosheets are developed and exhibit greatly enhanced pseudocapacitance via fast dual-ion-involved redox reactions with both large specific capacity and fast charge/discharge capability. By using an aqueous Na SO electrolyte, the oxygen-vacancy-tuned Fe O surface greatly enhances the absorption of SO anions that majorly increase the surface pseudocapacitance. Significantly, the Fe O -based electrode delivers a high specific capacity of 749 C g at 5 mV s and retains 290 C g at an ultrahigh scan rate of 3.2 V s . With a novel dual-electrolyte design, a 2 V Fe O /Na SO //MnO /Na SO asymmetric supercapacitor is constructed, delivering a high energy density of 75 W h kg at a power density of 3125 W kg .