Unlocking fast and stable Na storage in Na4Fe3(PO4)2P2O7 cathodes by diffusion kinetics optimization

Na₄Fe₃(PO₄)₂P₂O₇ has attracted considerable attention due to its promising electrochemical performance, low cost and environmental benefits. Nevertheless, the inferior intrinsic conductivity and poor Na⁺ diffusion kinetics restrict its rate and fast-charging performance. To address the inherent limitations of Na₄Fe₃(PO₄)₂P₂O₇, a Cu²⁺-doping strategy is adopted to enhance the dynamics. The incorporation of Cu²⁺ constructs rapid sodium diffusion channels, thereby enabling ultra-fast Na⁺ transport. Electrochemical experiments demonstrate that the Na⁺ diffusion coefficient of Na₄Fe_2.9Cu_0.1(PO₄)₂P₂O₇ in the plateau region has been significantly improved. Furthermore, density functional theory calculation shows that the incorporation of Cu²⁺ provides additional hybridization energy levels and effectively reduces the bandgap, enhancing the electron diffusion kinetics. Benefiting from the accelerated electrons/ions diffusion, the optimized Na₄Fe_2.9Cu_0.1(PO₄)₂P₂O₇ delivers excellent rate performance (79.43 mAh g⁻¹ at 50 C) and remarkable fast-charging performance (103.36 mAh g⁻¹ at 5 C/1 C with 98.78 % capacity retention after 300 cycles). In situ X-ray diffraction reveals the highly reversible Na⁺ insertion/extraction mechanism in Na₄Fe_2.9Cu_0.1(PO₄)₂P₂O₇ during the charging/discharging processes. More meaningfully, the outstanding electrochemical characteristics of the kilogram-scale Na₄Fe_2.9Cu_0.1(PO₄)₂P₂O₇ sample in large-capacity pouch cell exemplify the scalability and superiority of our synthesis strategy. This study contributes to the development of high-rate and fast-charging cathodes for practical sodium-ion batteries.