Probing Phase Formation and Structural Transformations in Sodium Extraction and Insertion of NaFe1- yMnyPO4 through First-Principles Calculations

Abstract

Manganese (Mn) substitution is a widely explored strategy aimed at sustainably enhancing the energy density of iron (Fe)-based electrode materials by taking advantage of the higher redox potential of the former. However, excessive Mn content can lead to detrimental effects, offsetting the expected improvements. In experimental studies, triphylite NaFe_0.8Mn_0.2PO₄ has been identified as an optimal composition with enhanced electrochemical performance compared to that of its parent phase NaFePO₄. Higher Mn contents result in a loss of capacity and increased voltage hysteresis. In this study, density functional theory (DFT) calculations were employed to investigate the phase stability upon desodiation of Mn-poor and -rich Na_xFe_1-yMn_yPO₄ compositions. Our findings reveal distinct stability behaviors in antagonistic systems Na_xFe_0.75Mn_0.25PO₄ and Na_xFe_0.25Mn_0.75PO₄, where the presence of Na-vacancies and charge orderings appear to influence phase stability. In addition, the number of intermediate phases throughout the desodiation process is identified as a crucial factor in buffering the volume changes. This work sheds light on the superior electrochemical performance of lightly Mn-substituted phases and unveils a key parameter for designing future electrode materials with improved performance.

Figure 1

Convex hulls representing the formation energies calculated for (A) the Na_xFe_0.75Mn_0.25PO₄ phases and (B) the Na_xFe_0.25Mn_0.75PO₄ phases. The four ground states identified in each hull are indicated with the labels H1_X (X = A, B, C, D) and H2_X (X = A, B, C, D), respectively. The points marked as H1_Y and H2_Y correspond to two unstable structures calculated for x = 0.67.

Figure 2

Representation of the superstructures determined for the four ground states of the Na_xFe_0.75Mn_0.25PO₄ system upon desodiation (x = 0.25, 0.5, 0.67, and 0.83). Red, brown, and white balls represent the O atoms, Na⁺ cations, and Na vacancies, respectively. Fe²⁺, Fe³⁺, and Mn²⁺ are localized with dark green, light green, and dark blue octahedra, as deduced from the Bader and BVS analyses of the structures. The charge orderings of Fe³⁺ (light green octahedra) with Na vacancies (white balls) are highlighted with black arrows.

Figure 3

Representation of the superstructures determined for the four ground states of the Na_xFe_0.25Mn_0.75PO₄ system upon desodiation (x = 0.5, 0.67, 0.75, and 0.83). Red, brown, and white balls represent O atoms, Na⁺ cations and Na vacancies, respectively. Fe²⁺, Fe³⁺, Mn²⁺, and Mn³⁺ are localized with dark green, light green, dark blue, and light blue octahedra, as deduced from the Bader and BVS analyses of the structures. The charge orderings of Fe³⁺ (light green octahedra) and Mn³⁺ (light blue octahedra) with Na vacancies (white balls) are highlighted with black arrows.

Figure 4

Distortion indices of the MnO₆ and FeO₆ octahedra for both Na_xFe_0.75Mn_0.25PO₄ and Na_xFe_0.25Mn_0.75PO₄ compositions upon desodiation.

Figure 5

Equilibrium voltages calculated between the different ground states identified in the convex hulls upon desodiation in the (A) Na_xFe_0.75Mn_0.25PO₄ and (B) Na_xFe_0.25Mn_0.75PO₄ systems.

Figure 6

Relative volume changes between the normalized unit cells of the ground states identified in both Na_xFe_0.75Mn_0.25PO₄ (green points) and Na_xFe_0.25Mn_0.75PO₄ (blue points) compositions during the desodiation process. The dashed lines serve as guides for the eye between each point. The main volumes mismatches observed in the range 0 < x < 0.5 are indicated in the figure.