Surface Mn-Enriched Doping Increasing Local Electron Concentration of Na4Fe3(PO4)2P2O7 Cathodes for Enhanced Sodium Storage

Abstract

A NASICON-type Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode material was successfully synthesized using a sand grinding-spray drying method. Different doping strategies can impart distinct modifications to materials, with surface Mn-rich doping (SD) being particularly effective. On one hand, the surface enrichment layer can effectively mitigate the volumetric fluctuations of particles, thereby reducing the internal stress and enhancing the cyclic stability. More importantly, the enrichment of the Mn in the particle surface layer provides an increased number of free electrons. This elevates the local electron concentration within the material, fosters greater overlap in the wave functions of electrons, and strengthens the interactions between electrons. The higher energy state of electrons due to increased transition propensity enhances the material's electronic conductivity. As a consequence, the band gap of SD material has decreased from 0.72 eV to 0.45 eV, and the electronic conductivity has increased from 6.0 μS·cm⁻¹ to 21.8 μS·cm⁻¹. The as-optimized SD sample displays both outstanding rate performance (110.8 mAh·g⁻¹ and 99.0 mAh·g⁻¹ at 0.1 C and 5 C, respectively) and excellent cycling stability (88.7% of capacity retention after 1500 cycles at 1 C). The study highlights that the choice of doping methods is equally crucial for the performance of NFPP materials.