Surface Pseudocapacitive Mechanism of Molybdenum Phosphide for High‐Energy and High‐Power Sodium‐Ion Capacitors

Abstract Sodium‐based energy storage technologies are potential candidates for large‐scale grid applications owing to the earth abundance and low cost of sodium resources. Transition metal phosphides, e.g. MoP, are promising anode materials for sodium‐ion storage, while their detailed reaction mechanisms remain largely unexplored. Herein, the sodium‐ion storage mechanism of hexagonal MoP is systematically investigated through experimental characterizations, density functional theory calculations, and kinetics analysis. Briefly, it is found that the naturally covered surface amorphous molybdenum oxides layers on the MoP grains undergo a faradaic redox reaction during sodiation and desodiation, while the inner crystalline MoP remains unchanged. Remarkably, the MoP anode exhibits a pseudocapacitive‐dominated behavior, enabling the high‐rate sodium storage performance. By coupling the pseudocapacitive anode with a high‐rate‐battery‐type Na 3 V 2 O 2 (PO 4 ) 2 F@rGO cathode, a novel sodium‐ion full cell delivers a high energy density of 157 Wh kg −1 at 97 W kg −1 and even 52 Wh kg −1 at 9316 W kg −1 . These findings present the deep understanding of the sodium‐ion storage mechanism in hexagonal MoP and offer a potential route for the design of high‐rate sodium‐ion storage materials and devices.