Carbon-Polytetrahydrofuran Double-Coated Na₃V₂(PO₄)₂F₃Submicron-Composite as High-Energy/Power Cathode Material for Sodium Metal Battery

A carbon-polytetrahydrofuran double-coated Na 3 V 2 (PO 4 ) 2 F 3 composite has been prepared by synthesis of carbon coated Na 3 V 2 (PO 4 ) 2 F 3 followed by in situ polymerization of tetrahydrofuran on its surface. Thermal gravimetry analysis, Fourier transform infrared spectrum and high-resolution transmission electron microscopy are used to confirm the encapsulation structure of the inner carbon layer and the outer polytetrahydrofuran layer on Na 3 V 2 (PO 4 ) 2 F 3 . Electrochemical tests show that electrochemical performance and electronic conductivity of the composite are significantly improved due to the polytetrahydrofuran coating, and a reversible capacity of 123.6 mAh g −1 at a moderate 2 C rate is obtained with a high capacity retention of 98.5% after 250 cycles. Even at an extremely high rate of 10 C the discharge capacity is still as high as 115.8 mAh g −1 with a capacity retention of 80% over 400 cycles. Interestingly, sodium ion distribution in augmented triangular prismatic sites of Na 3 V 2 (PO 4 ) 2 F 3 is not unchanged, but shifts from the high energy Na(2)' site to the low energy Na(3)' site after a certain cycle, thus Na 3 V 2 (PO 4 ) 2 F 3 could obtain a higher energy density. The full cell coupled with the composite cathode and Na metal anode constructed with Na-Sn alloy substrate exhibits excellent rate discharge capability and good cycling stability. The nano-level carbon-PTHF double-coating developed in this work could provide a new way to improve Na + -storage and rate performance of Na-based materials.