Effects of Bi doping on the electrochemical performance of Na3V2(PO4)3F3 cathode material for sodium ion batteries

Polyphosphate, as the cathode material of sodium ion battery(SIB) has the advantages of good structural stability and long service life, but suffer from poor conductivity and low specific capacity. The doping of heteroatom and coating of carbon are considered to be two effective measures to overcome its shortcomings. In this work, the Bismuth(Bi)-doped and carbon-coated materials Na₃V_2-xBi_x(PO₄)₂F₃/C with various Bi³⁺ doping levels(x = 0.03,0.05,0.07) were prepared by a facile sol-gel method combined high temperature calcination. The effect of Bi³⁺ doping on the electrochemical properties was systematically investigated. The Na₃V_1.95Bi_0.05(PO₄)₃F₃/C showed the best electrochemical performance with the specific capacities of 107.4, 94.3, 92.4, 86.2 mAh·g⁻¹ at 0.1 A·g⁻¹(0.78C), 0.2 A·g⁻¹(1.56C), 0.5 A·g⁻¹(3.9C), 1.0 A·g⁻¹(7.8C) respectively, and 90.4% of specific capacity was retained after 100 charge/discharge cycles, which has a greatly increase compared with the Na₃V₂(PO₄)₃F₃ material. This is attribute to the improving of the conductivity, the diffusion capacity and the structural stability of the material by Bi-doping and carbon coating.