Lattice Regulation Boosts Working Voltage and Energy Density of Na_3.12Fe_2.44(P₂O₇)₂ Cathode for Sodium‐Ion Batteries

Abstract Pyrophosphate Na_3.12Fe_2.44(P₂O₇)₂ (NFPO) has triggered widespread attention due to its high safety and robust framework. However, it suffers from low energy density due to the limited specific capacity and partial low voltage plateau (≈2.5 V vs Na⁺/Na). Regulating such low voltage plateau can be a promising way to increase the energy density of NFPO. As revealed by the systematic characterizations and theoretical calculations, the low‐voltage plateau results from the Fe1 site (corner‐sharing mode) redox reaction accompanied by Na4 insertion/extraction process. Accordingly, in this work, a precise Ti³⁺ doping strategy is proposed to selectively occupy the Fe1 site based on the energy effect, thus boosting the energy density from 277.5 to 307.7 Wh kg⁻¹ due to the improved reversible capacity (109.8 mAh g⁻¹) and regulated voltage plateau. Reversible structural evolution with very tiny volume variation (3%) due to the strengthened local environment (Ti─O bonds) promises outstanding cycling durability (90.3% capacity retention over 4000 cycles). Moreover, fast reaction kinetics and low diffusion energy barriers achieved by introducing Ti³⁺ contribute to the excellent rate capability. This lattice regulation strategy enables NFPO cathode with superior performance and can provide deep guidelines for designing advanced polyanion cathodes.