Synergistic Lattice Confinement and Local Bond Anchoring Strategy toward a Phase-Pure Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathode with Robust Lattice Framework for Sodium-Ion Batteries

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 is a potential cathode for sodium-ion batteries. However, its low electronic conductivity and insufficient Na + diffusion kinetics severely hinder practical application. Herein, this study proposes a lattice confinement-coupled local bond anchoring strategy, where the profound overlap between the vacant t 2g (d 0 ) orbitals of Ti 4+ and O 2p orbitals within the confined space enhances π-bond coupling hybridization. This enables rapid electron delocalization through the Ti–O–Fe network and intrinsically improves the electrical conductivity. Simultaneously, rigid [TiO 6 ] units anchor the lattice strain, confining it to microscopic regions and suppressing destructive macroscopic deformation. As a result, the Na 3.90 Fe 2.95 Ti 0.05 (PO 4 ) 2 P 2 O 7 retained 86.2% of its capacity after 2000 cycles at 10 C. Furthermore, at −40 °C, it delivers a reversible capacity of 90.43 mAh·g –1 at 0.5 C (vs 91.88% of room temperature). This demonstrates that the lattice-confinement-based design paradigm successfully unlocks the immense potential of sodium-ion batteries.