New Dimorphs of Na5V(PO4)2F2 as an Ultrastable Cathode Material for Sodium-Ion Batteries

In this work, a new fluorophosphate cathode material Na5V(PO4)(2)F-2 for sodium-ion batteries is synthesized by a facile synthetic strategy for the first time. X-ray diffraction analyses indicate that the novel material adopts two space groups (i.e., dimorphs): an orthorhombic form (space group (SG): Pbca) at high temperature (800 degrees C) and a trigonal form (SG: P3) at low temperature (600 degrees C). Both of them have a layered structure and could reversibly (de)intercalate one sodium with a V3+/V4+ redox couple. The new dimorphs also exhibit a high operating voltage due to the induction effect of fluorine, i.e., 3.4 V for trigonal Na5V(PO4)(2)F-2 (t-NVPF) and 3.5 V for orthorhombic Na5V(PO4)(2)F-2 (o-NVPF), respectively. In addition, they both exhibit outstanding cyclic stability, showing high capacity retentions of 86.5% and 92.6% after 1000 cycles for o-NVPF/C and t-NVPF at 136 mA g(-1), respectively. On the basis of ex situ XRD and DFT calculations, the excellent cycling performance of NVPF compounds such as t-NVPF is attributed to their robust polyanion framework, with fewer volume changes during reversible Na+ (de)intercalation processes.