Understanding the Electrochemical Mechanisms Induced by Gradient Mg²⁺ Distribution of Na-Rich Na_{3+ x}V_{2- x}Mg_x(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.