An Efficient Evaluation of F-doped Polyanion Cathode Materials with Long Cycle Life for Na-Ion Batteries Applications.

A series of Na V(PO F ) (x = 0, 0.1, 0.15 and 0.3) polyanion cathode materials are synthesized via a sol-gel method. The optimal doping concentration of F in NaV(PO) is 0.15 mol %. By neutron powder diffraction data, the chemical composition of as-synthesized material is NaV(POF). The half-cell of NaV(POF) cathode exhibits a stable discharge capacity of 103 mAh g and 93% of capacity retention over 250 cycles without decay at 0.1 A g, which is higher than that of bare NaV(PO) (98 mAh g). The high rate capability of NaV(POF) is also dramatically enhanced via increase the conductivity of host material by F-doping. Moreover, the symmetrical Na-ion full-cell is fabricated using NaV(POF) as cathode and anode materials. It is achieved that the good reversibility and superior cycling stability about 98% of capacity retention with ~100% of coulombic efficiency at 1.0 A g throughout 1000 cycles. These results demonstrate that the optimal amount of NaV(POF) is a distinctive potential candidate for excellent long-term cyclic stability with high rate low-cost energy storage applications.