Mn/Zn co-doping regulates the structure and electrochemical properties of Na4Fe3(PO4)2P2O7: A synergistic strategy for enhancing electronic conductivity and cycling stability.

Abstract

Na4Fe3(PO4)2P2O7 (NFPP), as a representative sodium-ion battery cathode material, has attracted extensive attention for large-scale energy storage owing to its outstanding cycling stability and cost advantages. However, its poor electronic conductivity and limited practical capacity remain major obstacles to commercialization. Herein, an Mn/Zn dual-cation regulation strategy targeting Fe sites is developed to modulate the NFPP framework. Structural characterization and electrochemical tests show that Mn doping not only increases the phase purity but also expands the NFPP lattice, which helps improve diffusion kinetics. In addition, the introduction of the Mn2+/Mn3+ redox couple during charging and discharging adjusts the local electronic structure. All of these factors together enhance the capacity and rate performance of NFPP materials. Meanwhile, the further incorporation of Zn2+ stabilizes the NFPP framework and facilitates charge transfer by mitigating lattice strain and optimizing the local coordination environment. Benefiting from the synergy between the enhanced electronic conductivity achieved through Mn doping and the improved structural stability during charge-discharge cycling achieved through Zn doping, the optimized NFPP-Z2M4 cathode delivers 119.8 and 89.0 mAh g-1 at 0.1C and 10C, respectively, while maintaining 81.12% capacity retention after 1000 cycles at 10C. This work establishes dual-cation doping as an effective design principle that combines enhanced electronic conductivity with improved cycling stability, providing new opportunities for the development of practical, high-performance polyanionic cathode materials for sodium-ion batteries.

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