Co-Manipulation of Ultrafine Nanostructure and Uniform Carbon Layer Activates Maricite-Structured NaFePO as a High-Performance Cathode for Sodium-Ion Batteries.

Sodium iron phosphate (NaFePO) has attracted significant attention because of its high theoretical capacity (155 mA h g), remarkable structural stability, and abundant elemental composition. However, the electrochemical reversibility of maricite NaFePO is generally considered inactive. Herein, a nanoengineering strategy to activate the electrochemical activity of maricite NaFePO is presented. This approach involves the construction of ultrasmall maricite NaFePO nanoparticles encapsulated within an ultrathin carbon layer (denoted as m-NFP@C), which greatly improves the electrochemical properties of the material. Notably, the optimized m-NFP@C nanoparticles exhibit an impressive reversible capacity of 101.4 mA h g after 100 cycles at a current density of 20 mA g, demonstrating a remarkable capacity retention of 90.5%. Furthermore, when coupled with the bismuth-carbon microfoam-like compound (Bi@NC-MF) anode, the fabricated sodium-ion full cell exhibits exceptional cycling stability with a capacity retention of 90.6% over 250 cycles. The remarkable electrochemical performance of this material can be attributed to its excellent structural stability, ultrafine nanostructure, and uniform carbon coating, which effectively shorten the Na diffusion pathways, prevent the aggregation and fragmentation of nanoparticles, and enhance electronic conductivity. This work is anticipated to open up a new route for activating maricite NaFePO and advancing the development of polyanion-type electrode materials.