Dual-site defects engineering to eliminate impurities and optimize reversible reaction kinetics of Na4Fe3(PO4)2P2O7 cathode for superior performance sodium ion batteries

Abstract

Na4Fe3(PO4)2P2O7 is a prominent polyanionic material widely studied as a cathode for sodium-ion batteries, valued for its stable cycling performance and cost-effectiveness. However, the sluggish diffusion kinetics of Na+ associated with electrochemically inert NaFePO4 impurities during synthesis strictly limit the rate performance and energy density of Na4Fe3(PO4)2P2O7. In this study, dual-site defects engineered Na4-2xFe3-1.5yLay(PO4-xBrx)2P2O7 cathode materials were synthesized using a facile mechanical activation method, by introducing trace amounts of LaBr3 as additive. Fe defects originating from La doping eliminate the maricite-NaFePO4 inert impurities and Na defects stemming from Br doping optimize the microchemical valence states and ion transport kinetics. The density functional theory demonstrates that Fe/Na dual-site defects in the lattice of Na4Fe3(PO4)2P2O7 reduce band gap and facilitate Na+ migration passageway, thereby leading to a superior rate capability and stable sodium storage performance. Moreover, the sodium storage mechanism of the dual-site defects engineered Na4Fe3(PO4)2P2O7 cathode material is revealed. The optimal dual-site defects engineered cathode sample delivers excellent rate performance (55.2 mAh g-1 at 50 C) and long cycling stability (capacity retention of 93% after 2000 cycles at 10 C). This study provides a promising strategy for engineering dual-site defects to synthesize impurities-free Na4Fe3(PO4)2P2O7 cathode material with superior electrochemical performance.