Unlocking iron-based mixed-phosphate cathode for sodium-ion batteries through off-stoichiometry

The off-stoichiometric iron-based phosphate (Na3.12Fe2.44(P2O7)2, denoted as Na3.12) as a low cost and high structure stability cathode material has been widely studied for sodium-ion batteries (SIBs). However, the lower theoretical specific capacity (117 mAh·g−1) has seriously limited its practical application. In this work, we incorporate varying proportion of sodium-iron phosphate (NaFePO4) into the Na3.12 to form a series of new high specific capacity mixed-phosphates Na3.12+xFe2.44+x(P2O7)2(PO4)x cathode materials for SIBs. After optimizing the introduction amount of NaFePO4 into Na3.12, the practical reversible of Na3.12+xFe2.44+x(P2O7)2(PO4)x increased from 92 to 125.2 mAh·g−1. The nano-size Na5.12Fe4.44(P2O7)2(PO4)2 cathode material shows a reversible specific capacity of 125.2 mAh·g−1 at 0.1 C in SIBs. Even at 60 C, it still exhibits a reversible specific capacity of 93.3 mAh·g−1 and keeps a capacity retention ratio of 87% after 3000 cycles at 20 C. Thereby, we present a novel approach to design a series of off-stoichiometric mixed-phosphates cathode materials for SIBs.