Vacancy Engineering Enables Ultralow Voltage Hysteresis in NASICON Cathodes for Mg-Metal Batteries

Rechargeable Mg-metal batteries are attractive owing to their high volumetric capacity, safety, and resource abundance, yet their development is limited by severe polarization and sluggish cathode kinetics. Herein, a cation-vacancy engineering strategy is achieved through Mg substitution in NASICON-type NaTi₂(PO₄)₃. The optimized Mg_0.25Na_0.5Ti₂(PO₄)₃ cathode exhibits low voltage hysteresis (0.05 V), high rate capability (87% capacity retention at 500 mA g⁻¹), and long-term cycling stability (81% capacity retention after 500 cycles). A proof-of-concept pouch cell delivers a low polarization voltage of 0.067 V. Mechanistic investigations reveal a Mg−Na hybrid-ion storage mechanism, in which Na⁺ predominantly governs the reversible cathode reaction while Mg metal undergoes reversible stripping/plating at the anode. Vacancy engineering effectively regulates ion transport and structural reversibility, enabling highly stable electrochemical operation. This work demonstrates vacancy engineering as an effective strategy for developing high-performance NASICON cathodes for Mg-metal battery systems.