Temperature‐dependent battery performance of a Na3V2(PO4)2F3cathode and in‐situ heat generation on cycling

Excellent structural stability, high operating voltage, and high capacity have made Na 3 V 2 (PO 4 )F 3   a promising cathode material for sodium-ion batteries. However, high-temperature battery performances and heat generation measurements have not been systematically reported yet. Carbon-coated Na 3 V 2 (PO 4 )F 3  @MWCNT (multi-walled carbon nanotube) samples were fabricated by a hydrothermal-assisted sol-gel method and the electrochemical performances were evaluated at three different temperatures (25, 45, and 55°C). The well-crystallized Na 3 V 2 (PO 4 )F 3  @MWCNT samples exhibited good cycling stability at both low and high temperatures; they delivered an initial discharge capacity of 120-125 mAhg -1 at a 1C rate with a retention of 53% capacity after 1,400 cycles with 99% columbic efficiency. The half-cell delivered a capacity of 100 mAhg -1 even at a high rate of 10C at room temperature. Furthermore, the Na 3 V 2 (PO 4 )F 3  @MWCNT samples showed good long-term durability; the capacity loss was an average of 0.05% per cycle at a 1C rate at 55°C. Furthermore, ionic diffusivity and charge transfer resistance were evaluated as functions of state of charge, and they explained the high electrochemical performance of the Na 3 V 2 (PO 4 )F 3  @MWCNT samples. In-situ heat generation measurements revealed reversible results upon cycling due to the high structural stability of the material. Excellent electrochemical performances were also demonstrated in the full-cell configuration with hard carbon as well as antimony Sb/C anodes.