Carbon-Coated Maricite NaFePO₄ Nanospheres Embedded in Graphene Nanosheets as Cathodes for Sodium-Ion Batteries

Sodium iron phosphate (NFP) is noted for its high capacity (154 mA h g^-1), safety, and stability. However, the more stable maricite phase is generally deemed inactive for electrochemistry due to restricted sodium ion pathways. Herein, carbon-coated NaFePO₄ nanospheres embedded in graphene nanosheets (NFP@PPy@rGO) have been synthesized by an in situ oxidation polymerization process. First, Fe³⁺ ions are adsorbed on the surface of GO by electrostatic field forces and pyrrole is polymerized by in situ oxidation of these surface-bound Fe³⁺ ions, and then the NFP@PPy@rGO composite is formed after carbonization. The in situ carbon coating not only enhances the electrical conductivity of NFP@PPy@rGO nanospheres but also inhibits the growth of spherical size, which can effectively shorten the ion and electron transport path of the active nanosphere itself. Meanwhile, rGO nanosheets form a three-dimensional conductive network that accelerates electron transport between nanospheres and enhances electrolyte wetting. The NFP@PPy@rGO electrode exhibits remarkable electrochemical performance, including a high reversible capacity of 126.7 mA h g^-1 at 10 mA g^-1 and excellent cycling performance. This study demonstrates the feasibility of improving the electrochemical performance of iron-based polyanion-type cathode materials by the proposed in situ oxidation polymerization method.