© 2018 American Chemical Society. Metal-ion doping can improve the electrochemical performance of Na 3 V 2 (PO 4 ) 3 . However, the reason for the enhanced electrochemical performance and the effects of cation doping on the structure of Na 3 V 2 (PO 4 ) 3 have yet been probed. Herein, Mg 2+ is doped into Na 3 V 2 (PO 4 ) 3 /C according to the first-principles calculation. The results indicate that Mg 2+ prefers to reside in the V site and an extra electrochemical active Na + is introduced to the Na 3 V 2 (PO 4 ) 3 /C crystal to maintain the charge balance. The distribution of Mg 2+ in the particle of Na 3 V 2 (PO 4 ) 3 /C is further studied by electrochemical impedance spectroscopy. We find that the highest distribution of Mg 2+ on the surface of the particles leads to facile surface electrochemical reactions for Mg 2+ -doped samples, especially at high rates.