Design of Biphasic Iron-Based Phosphate Cathodes with TiO ₂ -Induced Carbon–Lattice Reinforcement for Sodium-Ion Batteries

To address the challenges of structural degradation, slow Na+ kinetics, and voltage hysteresis in Na4Fe3(PO4)2P2O7 (NFPP)-based cathodes, we propose a carbon–lattice synergistic reinforcement strategy by introducing nano-TiO2 into a previously developed biphasic NFPP–Na2FeP2O7 (NFPO) composite (NFPP&NFPO). TiO2 mediates the pyrolysis of glucose, promoting the strengthening of the carbon framework while guiding the cogrowth of NFPP and NFPO phases through a hard-template-like effect. This enables the preservation of the hollow microspherical morphology from the spray-dried precursor, shortens Na+ diffusion pathways, and enhances the mechanical stability. Meanwhile, the partial substitution of Ti4+ into the NFPP lattice via the Kirkendall effect induces lattice contraction and creates redox-active Ti sites. The reversible Ti4+/Ti3+ transformation introduces a spring-like effect that mitigates stress during phase transitions and suppresses impedance fluctuations. As a result, the optimized NFPP&NFPOTi-2 delivers superior electrochemical performance, including 65.44 mAh g–1 at 50 C, nearly zero capacity decay over 500 cycles at 2 C, and 90.74% capacity retention after 10,000 cycles at 20 C with a voltage hysteresis of only 1.12 V. This study presents a feasible and scalable modification strategy for developing high-power, long-life sodium-ion battery cathodes.