Mitigating Lattice Distortion of Na4Fe3(PO4)2P2O7 Cathodes at High Voltage for High-Capacity Na-Ion Batteries

The Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode exhibits great potential for low-cost, high-safety, and long-life Na-ion batteries, yet lattice distortion at a high voltage of >3.1 V easily causes irreversible Na-ion extraction/insertion in pentagonal pyramid position (Na4 site). Herein, we forecast the elemental doping site according to the deviation degree and then realize the successful occupation of Li ions in Na4 sites of NFPP. The density functional theory calculations and experimental results verify that the Li ions in Na4 sites are not involved in the de/sodiation process but effectively hinder the shift of Fe along the a-axis and the distortion of P₂O₇ dime with well-maintained Na-ion diffusion paths even under high operation voltages. Consequently, Li-doped NFPP delivers an ultrahigh initial charge capacity of 128.7 mAh g^–1 (theoretical value: 129 mAh g^–1) with a Coulombic efficiency of 87.9%. It also exhibits a superior capacity retention of 95.7% after 150 times at 1C with a predictively long-term cycle life of 80% after 5589 h. The increase in energy density of Fe-based phosphate cathodes is reckoned to further accelerate their large-scale applications in energy storage systems.