Journal
ACS APPLIED ENERGY MATERIALS
Volume 3, Issue 1, Pages 1181-1189Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsaem.9b02248
Keywords
Na-ion batteries; cathode material; Na5V(PO4)(2)F-2; dimorphs; structural evolution
Funding
- National Key Research and Development Program of China [2018YFB0905400, 2016YFB0901500]
- National Natural Science Foundation of China [21233004, 21428303, 21621091, 11874307]
- Spanish Ministry of Science and Innovation [MAT2017-84002-C2-1-R, PRX18/00463]
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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.
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