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

Abstract

Abstract The NASICON‐type Na 3 MnTi(PO 4 ) 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 MnO 6 symmetry, mitigating Jahn–Teller distortion. The low‐cost Fe 2+ was introduced into the NMTP lattice, forming the Na 3+2x MnTi 1‐x Fe x (PO 4 ) 3 system. DFT calculations, ex situ XANES, and ssNMR analyses reveal a synergistic mechanism involving reduced vacancy concentration and stabilized MnO 6 symmetry, increasing IASD formation energy and improving structural stability, effectively suppressing both voltage hysteresis and Jahn–Teller distortion. The optimized Na 3.2 MnTi 0.9 Fe 0.1 (PO 4 ) 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.