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

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 Na_4–2xFe_3–1.5yLa_y(PO_4-xBr_x)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.