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

Abstract

The off-stoichiometric iron-based phosphate (Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , denoted as Na 3.12 ) 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 (NaFePO 4 ) into the Na 3.12 to form a series of new high specific capacity mixed-phosphates Na 3.12+ x Fe 2.44+ x (P 2 O 7 ) 2 (PO 4 ) x cathode materials for SIBs. After optimizing the introduction amount of NaFePO 4 into Na 3.12 , the practical reversible of Na 3.12+ x Fe 2.44+ x (P 2 O 7 ) 2 (PO 4 ) x increased from 92 to 125.2 mAh·g −1 . The nano-size Na 5.12 Fe 4.44 (P 2 O 7 ) 2 (PO 4 ) 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.