Probing Phase\nFormation and Structural Transformations\nin Sodium Extraction and Insertion of NaFe<sub>1–<i>y</i></sub>Mn<sub><i>y</i></sub>PO<sub>4</sub> through First-Principles\nCalculat

Manganese\n(Mn) substitution is a widely explored strategy aimed\nat sustainably enhancing the energy density of iron (Fe)-based electrode\nmaterials by taking advantage of the higher redox potential of the\nformer. However, excessive Mn content can lead to detrimental effects,\noffsetting the expected improvements. In experimental studies, triphylite\nNaFe<sub>0.8</sub>Mn<sub>0.2</sub>PO<sub>4</sub> has been identified\nas an optimal composition with enhanced electrochemical performance\ncompared to that of its parent phase NaFePO<sub>4</sub>. Higher Mn\ncontents result in a loss of capacity and increased voltage hysteresis.\nIn this study, density functional theory (DFT) calculations were employed\nto investigate the phase stability upon desodiation of Mn-poor and\n-rich Na<sub><i>x</i></sub>Fe<sub>1–<i>y</i></sub>Mn<sub><i>y</i></sub>PO<sub>4</sub> compositions.\nOur findings reveal distinct stability behaviors in antagonistic systems\nNa<sub><i>x</i></sub>Fe<sub>0.75</sub>Mn<sub>0.25</sub>PO<sub>4</sub> and Na<sub><i>x</i></sub>Fe<sub>0.25</sub>Mn<sub>0.75</sub>PO<sub>4</sub>, where the presence of Na-vacancies and\ncharge orderings appear to influence phase stability. In addition,\nthe number of intermediate phases throughout the desodiation process\nis identified as a crucial factor in buffering the volume changes.\nThis work sheds light on the superior electrochemical performance\nof lightly Mn-substituted phases and unveils a key parameter for designing\nfuture electrode materials with improved performance.