Regulating Na/Mn Antisite Defects and Revitalizing Reversible Redox Reactions in Phosphate Cathodes

Na₃MnTi(PO₄)₃ (NMTP) represents an attractive cathode candidate for sodium-ion batteries, providing a low-cost and high-safety solution for energy storage systems. However, Mn²⁺ residing in the Na⁺ vacancy seriously hinders Na⁺ transportation, which significantly impedes NMTP from achieving the theoretical specific capacity. Herein, we introduce a cation gap-filling strategy via excess sodium incorporation to effectively suppress the Mn²⁺ occupation at Na sites, thereby increasing the Na⁺ vacancy concentration and promoting rapid Na⁺ diffusion kinetics. Cyclic voltammetry and galvanostatic intermittent titration technique tests further demonstrate the faster reaction kinetics of Na_3.5MnTi(PO₄)₃ (NMTP-Na0.5). The cathode-electrolyte side reactions are effectively inhibited through the introduction of additional sodium, as confirmed by HRTEM and XPS analyses. <i>In situ</i> XRD and density functional theory (DFT) calculations reveal reduced structural evolution and lower Na⁺ migration energy barriers upon tuning the sodium vacancy concentration. Consequently, the synthesized Na_3.5MnTi(PO₄)₃ (NMTP-Na0.5) exhibits exceptional electrochemical properties, achieving an ultrahigh capacity of 163.9 mAh g^-1 at 0.1 C. This performance ensures a stable energy output of approximately 500 × 10³ mWh kg^-1, along with a high rate capability and cycling stability. This work provides a viable pathway toward the development of high-energy-density electrode materials for next-generation grid-scale energy storage applications.