Uncovering the origin of surface-redox pseudocapacitance of molybdenum phosphides enables high-performance flexible sodium-ion capacitors

The flexible electrode based on surface-redox, pseudocapacitive material delivers high specific capacity and superior rate capability, which shows excellent advantages in achieving low-cost and high-performance flexible sodium ion capacitors (FSICs). However, a considerable challenge to be solved is to explore the origin and optimization of surface pseudocapacitance. Herein, we report freestanding MoP nanowires flexible films (MoP-NWs-FF) with high surface-redox pseudocapacitance, and its conductive frameworks facilitate electron/ion transport and achieve superior mechanical flexibility. According to X-ray photoelectron spectroscopy (XPS) and advanced scanning transmission electron microscopy (STEM) technology, the amorphous oxides on the surface of MoP nanograins are demonstrated, which is positively correlated with the sodium ion storage capacity. As a result, the optimized MoP-NWs-FF electrode delivers a high reversible specific capacity of 293 mAh g−1 at 0.1 A g−1, superior rate performance, and cycling stability. Besides, based on the MoP-NWs-FF anode and Na3V2(PO4)2/C flexible film cathode, the assembled fully FSIC exhibits a high power density of ∼ 4 kW kg−1 (with an energy density of 53 Wh kg−1), corresponding to a discharging time of 1 min. This work provides a fundamental insight into the origin of surface-redox pseudocapacitance and paves a new way to develop next-generation high-power flexible energy storage devices.