Toward Sustainable Sodium-Ion Storage via Local Structural Engineering of Nonstoichiometric Iron-Based Mixed Phosphate–Pyrophosphate Cathodes

Abstract The nonstoichiometric iron-based mixed phosphate (Na5.12Fe4.44(PO4)2(P2O7)2, N5.12FPP) has emerged as a promising cathode material for sodium-ion batteries (SIBs), benefiting from its low cost, intrinsic safety, and robust structural framework. However, its practical application remains limited by intrinsically low electronic conductivity, sluggish Na-ion transport kinetics, and insufficient long-term cycling stability. In this work, an aliovalent Ti-doping strategy is proposed to modulate the local crystal structure and electronic-state distribution of nonstoichiometric N5.12FPP materials via spray drying following by solid-state calcination method. After optimizing the Ti-doping level, the Na4.72Fe4.24Ti0.2(PO4)2(P2O7)2 (N5.12FPP-0.2Ti) cathode delivers a reversible capacity of 128.5 mAh g–1 at 0.1 C, retains 104.2 mAh g–1 even at 20 C, and maintains 90.05% of its initial capacity after 3000 cycles at 20 C in SIBs. Moreover, a full sodium-ion cell assembling with N5.12FPP-0.2Ti as the positive electrode and commercial hard carbon as the negative electrode achieves a maximum gravimetric energy density of 302.9 Wh kg–1 and power density of 30.4 W kg–1. Therefore, these results demonstrate that Ti-doping can reinforce the host framework, facilitate Na-ion transport, and improve the electrochemical performance of Fe-based mixed-phosphate cathode materials, highlighting its potential for advanced SIBs.