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. In situ 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.