Optimizing the Mg Doping Concentration of Na₃V_{2- x}Mg_x(PO₄)₂F₃/C for Enhanced Sodiation/Desodiation Properties

Na3V2(PO4)2F3 with a NASICON (Na-superionic conductor) structure is a promising cathode material for sodium-ion batteries (NIBs) due to its high-energy density and great cycling stability. However, its low conductivity leads to inferior rate capability, which impedes its practical application. Herein, we report the synthesis of carbon-coated Na3V2-xMgx(PO4)2F3 with various Mg2+ doping levels (x = 0, 0.01, 0.05, and 0.1) using a facile sol-gel method. The effects of Mg2+ doping on the material and electrochemical properties are systematically investigated. The X-ray diffraction peaks shift to higher angles, reflecting a lattice contraction with increasing Mg2+ content. Rietveld refinement reveals the Na-O, V-O, and P-O bond length values of various Na3V2-xMgx(PO4)2F3 samples. The optimal carbon-coated Na3V1.95Mg0.05(PO4)2F3 shows excellent rate capability of 80 mA h g-1 at 10 C; moreover, 88% of this capacity can be retained after 500 charge/discharge cycles with an average Coulombic efficiency of 99.9%. The superior performance can be attributed to (i) enhanced electronic conductivity, (ii) improved Na+ transport, (iii) reduced crystal and particle sizes, and (iv) increased structural stability due to Mg2+ doping.