Effects of Mg doping on the remarkably enhanced electrochemical performance of Na₃V₂(PO₄)₃ cathode materials for sodium ion batteries

Na₃V_2-xMg_x(PO₄)₃/C composites with different Mg²⁺ doping contents (x = 0, 0.01, 0.03, 0.05, 0.07 and 0.1) were prepared by a facile sol-gel method. The doping effects on the crystal structure were investigated by XRD, XPS and EXAFS. The results show that low dose doping of Mg²⁺ does not alter the structure of the material, and magnesium is successfully substituted for the vanadium site. The Mg doped Na₃V_2-xMg_x(PO₄)₃/C composites exhibit significant improvements on the electrochemical performance in terms of the rate capability and cycle performance, especially for the Na₃V_1.95Mg_0.05(PO₄)₃/C. For example, when the current density increased from 1 C to 30 C, the specific capacity only decreased from 112.5 mA h g^-1 to 94.2 mA h g^-1 showing very good rate capability. Moreover, even cycling at a high rate of 20 C, an excellent capacity retention of 81% is maintained from the initial value of 106.4 mA h g^-1 to 86.2 mA h g^-1 at the 50th cycle. Enhanced rate capability and cycle performance can be attributed to the optimized particle size, structural stability and enhanced ionic and electronic conductivity induced by Mg doping.