Titanium-Induced Lattice Engineering in Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ Cathodes Enabling Superior Power Density and Long Cycle Life for Na-Ion Batteries

Sodium-ion batteries (SIBs) have gained prominence as viable systems for large-scale energy storage, and their performance is critically determined by cathodes that govern capacity and long-term stability. Among them, Na4Fe3(PO4)2P2O7 (NFPP) features high structural stability and interconnected Na+ diffusion channels yet suffers from poor phase purity and low electronic conductivity. Herein, we report a Ti-doped and carbon-coated NFPP composed of microsized secondary spheres with greatly enhanced reaction kinetics and superior structure stability. The incorporation of Ti, with its smaller ionic radius, effectively elongates Na–O bonds to widen Na-ion transport channels, while strong Ti–O bonds reinforce the lattice and suppress the formation of impurity phases. Consequently, the optimized Ti-NFPP achieves a reversible capacity of 106.7 mAh g–1 at 0.1 C and exhibits an excellent rate performance of 92.8 mAh g–1 at 10 C, as well as outstanding long-term stability with 99.5% capacity retention after 1000 cycles at 5 C. Furthermore, density functional theory calculations are performed to reveal the role of Ti doping in regulating the electrochemical behavior of NFPP. This study demonstrates that the introduction of Ti with lattice engineering provides a robust route to high-performance NFPP cathodes for SIBs.