From Li to Na: Exploratory Analysis of Fe-Based Phosphates Polyanion-Type Cathode Materials by Mn Substitution

Both LiFePO₄ (LFP) and NaFePO₄ (NFP) are phosphate polyanion-type cathode materials, which have received much attention due to their low cost and high theoretical capacity. Substitution of manganese (Mn) elements for LFP/NFP materials can improve the electrochemical properties, but the connection between local structural changes and electrochemical behaviors after Mn substitution is still not clear. This study not only achieves improvements in energy density of LFP and cyclic stability of NFP through Mn substitution, but also provides an in-depth analysis of the structural evolutions induced by the substitution. Among them, the substitution of Mn enables LiFe_0.5 Mn_0.5 PO₄ to achieve a high energy density of 535.3 Wh kg^-1 , while NaFe_0.7 Mn_0.3 PO₄ exhibits outstanding cyclability with 89.6% capacity retention after 250 cycles. Specifically, Mn substitution broadens the ion-transport channels, improving the ion diffusion coefficient. Moreover, LiFe_0.5 Mn_0.5 PO₄ maintains a more stable single-phase transition during the charge/discharge process. The transition of NaFe_0.7 Mn_0.3 PO₄ to the amorphous phase is avoided, which can maintain structural stability and achieve better electrochemical performance. With systematic analysis, this research provides valuable guidance for the subsequent design of high-performance polyanion-type cathodes.