Mo2C Nanoparticles Embedded in Carbon Nanowires with Surface Pseudocapacitance Enables High‐Energy and High‐Power Sodium Ion Capacitors

Electrochemical sodium‐ion storage technologies have become an indispensable part in the field of large‐scale energy storage systems owing to the widespread and low‐cost sodium resources. Molybdenum carbides with high electron conductivity are regarded as potential sodium storage anode materials, but the comprehensive sodium storage mechanism has not been studied in depth. Herein, Mo2C nanowires (MC‐NWs) in which Mo2C nanoparticles are embedded in carbon substrate are synthesized. The sodium‐ion storage mechanism is further systematically studied by in/ex situ experimental characterizations and diffusion kinetics analysis. Briefly, it is discovered that a faradaic redox reaction occurs in the surface amorphous molybdenum oxides on Mo2C nanoparticles, while the inner Mo2C is unreactive. Thus, the as‐synthesized MC‐NWs with surface pseudocapacitance display excellent rate capability (a high specific capacity of 76.5 mAh g−1 at 20 A g−1) and long cycling stability (a high specific capacity of 331.2 mAh g−1 at 1 A g−1 over 1500 cycles). The assembled original sodium ion capacitor displays remarkable power density and energy density. This work provides a comprehensive understanding of the sodium storage mechanism of Mo2C materials, and constructing pseudocapacitive materials is an effective way to achieve sodium‐ion storage devices with high power and energy density. The amorphous molybdenum oxides on the surface of molybdenum carbide nanoparticles undergo a faradaic redox reaction during the sodiation and desodiation process, which exhibits notable pseudocapacitive behavior. A sodium ion capacitor based on pseudocapacitive Mo2C nanowires anode coupled with high‐rate battery‐type Na3V2 (PO4)2F@C cathode delivers a high energy and power density.