Insights into the elevated electrochemical performance and kinetic characteristics of magnesium-substituted Na3V2−xMgx(PO4)3/C with superior rate capability and long lifespan

Na₃V₂(PO₄)₃ has been pursued to be a prospective cathode for SIBs because of its outstanding structural stability. Nevertheless, the extensive application of Na₃V₂(PO₄)₃ is impeded by its poor electronic conductivity and inferior Na⁺ migration ability. Herein, a promising Mg-doped Na₃V_2−xMg_x(PO₄)₃/C composite is prepared by a facile carbon-thermal reduction route. The substitution of magnesium onto vanadium site downsizes the particle, providing shorter pathway for the migration of Na⁺ and electron. The introduction of Mg²⁺ generates beneficial holes to facilitate the electronic diffusion efficiently. A superior electrochemical performance of Na₃V_1.93Mg_0.07(PO₄)₃/C sample can be achieved due to the multiple synergetic effects. It delivers a high specific capacity of 113.5 mAh g⁻¹ at 0.1 C. A high reversible capacity of 95 mAh g⁻¹ can be obtained at 10 C rate, and the retention is 84.6% after 1000 cycles. Moreover, a comprehensive GITT analysis is conducted to give a better understanding of the elevated electrochemical properties for Na₃V_1.93Mg_0.07(PO₄)₃/C: The migration of Na⁺ suffers from the intense interactions arising from the phase transfer during the potential plateau (~ 3.4 V). The minimum values of D_Na+ at ~ 3.4 V for Na₃V_1.93Mg_0.07(PO₄)₃/C are one order magnitude higher than that of undoped sample, implying the improved kinetics from magnesium substitution.