Regulating Sodium Vacancy and Local Coordination Structure Enabled Stable Mn-Based NASICON Cathodes

The NASICON-type Na₃MnTi(PO₄)₃ (NMTP) cathode is a promising candidate for sodium-ion batteries due to low cost, high capacity, and energy density. However, voltage hysteresis (from Mn/Na2-vacancies intrinsic antisite defects, IASDs) and structural degradation (via Jahn-Teller distortion) limit its application. Herein, we propose a sodium vacancy and local coordination coupling strategy involving low-valent ion doping to trigger charge compensation, thereby reducing the initial Na vacancy concentration and activating additional Na2 sites to suppress IASDs formation. Furthermore, the reconstructed Mn─O coordination environment enhances MnO₆ symmetry, mitigating Jahn-Teller distortion. The low-cost Fe²⁺ was introduced into the NMTP lattice, forming the Na_3+2xMnTi_1-xFe_x(PO₄)₃ system. DFT calculations, ex situ XANES, and ssNMR analyses reveal a synergistic mechanism involving reduced vacancy concentration and stabilized MnO₆ symmetry, increasing IASD formation energy and improving structural stability, effectively suppressing both voltage hysteresis and Jahn-Teller distortion. The optimized Na_3.2MnTi_0.9Fe_0.1(PO₄)₃ cathode demonstrates exceptional electrochemical performance, including high specific capacity (174.2 mAh g^-1 at 0.1 C), outstanding rate capability (125.5 mAh g^-1 at 20 C), and long-term cycling stability (85% retention after 2000 cycles at 5 C). This work provides new insights into the design of high energy density, long-lifespan sodium-ion batteries through sodium vacancy and coordination engineering.