Construction of Ultrafine Bimetallic Mn-Fe Phosphide Embedded in Nitrogen-Doped 3D Carbon Shells and the Excellent Na-Ion Storage Performance.

Transition-metal phosphides (TMPs) possess high theoretical capacity and voltage, which make them promising candidates for anode materials in sodium-ion batteries (SIBs). However, TMP anode materials face challenges such as significant volume expansion and poor conductivity. In this study, we synthesized nanobimetallic manganese-iron phosphide (MnFeP@NC) encapsulated in hollow nitrogen-doped carbon shells using a two-step method. This structure effectively mitigates volume expansion, enhancing the cycling stability. The incorporation of manganese improves both the capacity and conductivity, allowing MnFeP@NC to achieve a specific capacity of 401.3 mAh g at a current density of 1 A g. Ex situ X-ray diffraction (XRD) and in situ electrochemical impedance spectroscopy (EIS) techniques were used to study the sodium storage mechanism and kinetics during the discharge/charge processes. Additionally, density functional theory (DFT) calculations were performed to analyze the synergistic effect of the bimetallic system on promoting charge transfer and reaction kinetics. In a full cell with sodium vanadate phosphate as the cathode material, the battery also exhibits a high specific capacity. This study highlights the great potential of MnFeP@NC composites in enhancing the performance of sodium-ion batteries.