Mn–Ni Foreign Ions Codoping Polyanionic Iron-Based Compounds of Na_3.12Fe_2.44(P₂O₇)₂ for Long-Life and High-Rate Sodium-Ion Battery Cathode

Sodium-ion batteries (SIBs) are poised to be applied in the field of large-scale energy storage due to their unique advantages (low cost and resourceful). The polyanionic iron-based materials of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 (NFPO) with an open three-dimensional framework and excellent thermal stability have been considered as outstanding cathodes for SIBs. However, challenges such as poor conductivity, electrolyte corrosion, and structural damage during cycling still need to be addressed. In this study, we designed a Mn- and Ni-codoped Na 3.12 Fe 2.44 (P 2 O 7 ) 2 @C (NFMNP) material, while Mn 2+ and Ni 2+ can enhance the intrinsic conductivity of the NFP material. On the other hand, they can improve the formation of the cathode–electrolyte interface membrane during cycling. Additionally, interactions among Mn, Ni, and Fe elements inhibit the strong electrostatic repulsion between Fe 3+ –Fe 3+, resulting in excellent rate performance and cycling stability. At room temperature, a high current density of 1 A/g (≈8 C) achieves a capacity of 97 mA h·g –1 (theoretical capacity = 117 mA h·g –1 ), maintaining the capacity without any decay over 300 cycles, which is significantly higher than that of the NFP (71.6%). Furthermore, after 1000 cycles, the capacity retention remains at 80.04%. This work demonstrates the effectiveness of ion doping and provides an efficient strategy for high-rate SIBs.