Effects of Mg doping on the remarkably enhanced electrochemical performance of Na3V2(PO4)3 cathode materials for sodium ion batteries†
Na3V2−xMgx(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 Na3V2−xMgx(PO4)3/C composites exhibit significant improvements on the electrochemical performance in terms of the rate capability and cycle performance, especially for the Na3V1.95Mg0.05(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.