Understanding the Electrochemical Mechanisms Induced by Gradient Mg2+ Distribution of Na-Rich Na3+xV2–xMgx(PO4)3/C for Sodium Ion Batteries

Metal-ion doping can improve the electrochemical performance of Na3V2(PO4)3. However, the reason for the enhanced electrochemical performance and the effects of cation doping on the structure of Na3V2(PO4)3 have yet been probed. Herein, Mg2+ is doped into Na3V2(PO4)3/C according to the first-principles calculation. The results indicate that Mg2+ prefers to reside in the V site and an extra electrochemical active Na+ is introduced to the Na3V2(PO4)3/C crystal to maintain the charge balance. The distribution of Mg2+ in the particle of Na3V2(PO4)3/C is further studied by electrochemical impedance spectroscopy. We find that the highest distribution of Mg2+ on the surface of the particles leads to facile surface electrochemical reactions for Mg2+-doped samples, especially at high rates.