Co-Manipulation of Ultrafine Nanostructure and Uniform Carbon Layer Activates Maricite-Structured NaFePO4 as a High-Performance Cathode for Sodium-Ion Batteries.

Sodium iron phosphate (NaFePO4) has attracted significant attention because of its high theoretical capacity (155 mA h g-1), remarkable structural stability, and abundant elemental composition. However, the electrochemical reversibility of maricite NaFePO4 is generally considered inactive. Herein, a nanoengineering strategy to activate the electrochemical activity of maricite NaFePO4 is presented. This approach involves the construction of ultrasmall maricite NaFePO4 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-1 after 100 cycles at a current density of 20 mA g-1, 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 NaFePO4 and advancing the development of polyanion-type electrode materials.