Defect Engineering and Na-Ion Transport in NaMnPO4: A Computational Perspective

Rechargeable sodium-ion batteries (SIBs) have attracted considerable attention owing to the natural abundance and accessibility of sodium. Maricite NaMnPO4, a phosphate-based cathode material with high theoretical capacity, suffers from blocked sodium-ion diffusion channels. In this study, atomistic simulations using pair potentials and density functional theory (DFT) are employed to investigate intrinsic defect mechanisms, sodium-ion migration pathways, and the role of dopant incorporation at Na, Mn, and P sites in generating Na vacancies and interstitials. Among the intrinsic defects, the Na–Mn anti-site cluster emerges as the most favorable, exhibiting a very low formation energy of 0.12 eV, while the Na Frenkel pair (1.93 eV) is the next most stable defect, indicating that sodium diffusion is primarily facilitated by vacancy formation. Nevertheless, sodium-ion mobility in NaMnPO4 remains limited, as reflected by the relatively high migration activation energy of 1.28 eV. Among the isovalent substitutions, K is predicted to be the most favorable dopant at the Na site, whereas Ca and Cu are the most favorable at the Mn site. Thallium is identified as a promising dopant at the Mn site for generating Na vacancies that facilitate Na-ion migration, while Ge substitution at the P site is predicted to enhance the sodium content in the material.