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

A series of Na_3-x V₂(PO_4-x F _x )₃ (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 Na₃V₂(PO₄)₃ is 0.15 mol %. By neutron powder diffraction data, the chemical composition of as-synthesized material is Na_2.85V₂(PO_3.95F_0.05)₃. The half-cell of Na_2.85V₂(PO_3.95F_0.05)₃ cathode exhibits a stable discharge capacity of 103 mAh g^-1 and 93% of capacity retention over 250 cycles without decay at 0.1 A g^-1, which is higher than that of bare Na₃V₂(PO₄)₃ (98 mAh g^-1). The high rate capability of Na_2.85V₂(PO_3.95F_0.05)₃ is also dramatically enhanced via increase the conductivity of host material by F-doping. Moreover, the symmetrical Na-ion full-cell is fabricated using Na_2.85V₂(PO_3.95F_0.05)₃ 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^-1 throughout 1000 cycles. These results demonstrate that the optimal amount of Na_2.85V₂(PO_3.95F_0.05)₃ is a distinctive potential candidate for excellent long-term cyclic stability with high rate low-cost energy storage applications.