A new sodium ferrous orthophosphate Na x Fe4(PO4)3 as anode materials for sodium-ion batteries

A new sodium ferrous orthophosphate Na x Fe 4 (PO 4 ) 3 (1.1 ≤  x  ≤ 1.2) with P 2 1 / n symmetry has been successfully synthesized via a solid-state reaction for the first time. The structure consists of FeO 5 pyramids, FeO 6 octahedron, and PO 4 tetrahedra which form large six-sided tunnels along \( \vec{b} \) direction and three- or four-sided tunnels running along \( \vec{a} \) direction. When evaluated as the anode materials for sodium-ion batteries, the first discharge and charge capacities of 553 and 264 mAh g −1 for Na 1.1 Fe 4 (PO 4 ) 3 /C and 583 and 315 mAh g −1 for Na 1.2 Fe 4 (PO 4 ) 3 /C, respectively, can be obtained at 10 mA g −1 current density. Compared to the Na 1.1 Fe 4 (PO 4 ) 3 /C, Na 1.2 Fe 4 (PO 4 ) 3 /C exhibits a better cycling performance, where the charge capacity of about 100 mAh g −1 can still be maintained after 15 cycles. The electrode kinetics are investigated further by the electrochemical impedance spectroscopy.