Carbon-Polytetrahydrofuran Double-Coated Na3V2(PO4)2F3 Submicron-Composite as High-Energy/Power Cathode Material for Sodium Metal Battery

A carbon-polytetrahydrofuran double-coated Na3V2(PO4)2F3 composite has been prepared by synthesis of carbon coated Na3V2(PO4)2F3 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 Na3V2(PO4)2F3. 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 Na3V2(PO4)2F3 is not unchanged, but shifts from the high energy Na(2)' site to the low energy Na(3)' site after a certain cycle, thus Na3V2(PO4)2F3 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.