Flexible Na _3.5 Fe _2.5 (PO ₄ ) _1.5 P ₂ O ₇ /C Nanofiber Membrane Cathodes and the Derived Flexible Carbon Anodes for Sodium Ion Batteries

With the advent of foldable and wearable electronic devices, flexible sodium ion batteries are being rapidly developed as one of the most appealing power sources. Herein, a flexible Na_3.5Fe_2.5(PO₄)_1.5P₂O₇/C nanofiber membrane is prepared for application as the cathode of SIBs. Benefited from its excellent electronic conductivity and fast Na⁺ migration, the flexible cathode delivers a high reversible capacity of 83.9 mAh·g^-1 and initial Coulombic efficiency of 96.0% at 0.2 C, outstanding rate performance (50 mAh·g^-1 at 10 C), and superior cycling stability (93.5% capacity retention over 300 cycles at 0.2 C), based on the membrane mass. Furthermore, the Na_3.5Fe_2.5(PO₄)_1.5P₂O₇/C cathode exhibits stable cycling with capacity retention of 82.3% after 1500 cycles at 3 C. The reversible Na⁺ intercalation/deintercalation and Fe²⁺/Fe³⁺ transformation have been demonstrated by in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy, respectively. Meanwhile, after HCl etching and annealing, a flexible carbon nanofiber membrane is derived and applied as the anode of SIBs. A coin-type full cell coupling the flexible Na_3.5Fe_2.5(PO₄)_1.5P₂O₇/C cathode and the flexible carbon anode achieves 90.2% capacity retention after 1500 cycles at 1 C. Furthermore, a foldable full-cell prototype validates the potential of the electrospun flexible nanofiber membrane electrodes for flexible SIBs.