Pillar doping of Na4 site in Na4Fe3(PO4)2P2O7 alleviating structural evolution at high voltages for sodium storage

In this work, for the first time, it is demonstrated that during the insertion/extraction of Na ions, the structural evolution at the Na4 site at a voltage range of 3–4 V is a key factor for the capacity decay of Na₄Fe₃(PO₄)₂P₂O₇ (NFPP). Herein, a strategy of introducing columnar potassium ions at the Na4 site is proposed to address the aforementioned challenge. As a cathode material for sodium-ion batteries, the K_0.12Na_3.88Fe₃(PO₄)₂P₂O₇/C (K-NFPP) composite enhances the reversibility of Na4 extraction. Specifically, the K-NFPP exhibits an initial discharge capacity of 107.8 mAh g⁻¹ at a high current density of 5 C, with a capacity retention of 91.4% after 2000 cycles, outperforming the pristine NFPP material (81.1 mAh g⁻¹ and 67.1%). At 5 C, the K-NFPP also retains 81.5% of the reversible capacity at 0.1 C, whereas the NFPP only retains 68.3%. Moreover, the K-NFPP-based full-cell delivers an initial capacity of 110.1 mAh g⁻¹ at 1 C, with a capacity retention of 90% after 100 cycles. It is found that in comparison to K-doping of the Na1, Na2, and Na3 sites, K-doping at the Na4 site effectively optimizes the band gap and stabilizes the crystal structure, thereby reducing lattice changes of FeO₆ evolution during Na⁺ insertion/extraction. As a result, the introduction of columnar potassium ions significantly enhances the capacity contribution of the Na4 site, optimizes reaction kinetics, and effectively mitigates the capacity decay of NFPP cathodes. It is believed that this study offers a new entry point for the application of NFPP in high-voltage sodium storage.