Mg2+-driven triphasic reaction breaking kinetic barriers: Toward ultra-stable and ultrafast sodium-ion polyanionic cathodes

Abstract

Sodium-ion batteries are promising candidates for next-generation large-scale energy storage owing to their abundance and low cost. Biphasic intercalation reactions, constrained by kinetic limitations and structural instability, fundamentally restrict the rate capability and cycle life of sodium‑ion batteries. However, precise regulation of these reactions to enhance kinetics remains challenging. Here, we propose a strategy of atomic-scale phase engineering to activate the metastable state and achieve a three-phase reaction through precise Mg²⁺ doping at V sites in Na₃V₂(PO₄)₃. The Mg²⁺ occupancy promotes the exchange between Na1 and Na2 sites, thereby stabilizing a Na₂V₂(PO₄)₃ intermediate. First-principles calculations indicate that Mg²⁺ occupation facilitates charge redistribution by weakening Na-O electrostatic interaction, stabilizing the formation of Na₂V₂(PO₄)₃ phase. The optimized cathode exhibits ultrahigh capacity retention (84.5 % after 5000 cycles at 3.51 A g⁻¹), supports ultrafast charging within 120 s, and exceptional rate capability (96.2 mAh g⁻¹ at 4.68 A g⁻¹). This work establishes a universal route to unlock hidden reaction pathways by redefining the role of dopants in phase‑transition control.