Superstructure Engineering Enables NASICON‐Type Phosphate Cathodes with Increased Working Voltage and Energy Density

Abstract Na + Super Ionic CONductor (NASICON)‐type iron‐based phosphate cathode has attained extensive research interest due to its green, low cost, and superior rate capability for sodium‐ion batteries (SIBs). However, owing to strong Fe─O covalent character in the NASICON frameworks, the low Fe 2+ /Fe 3+ redox potential (<2.5 V vs Na + /Na) has led to an undesirable energy density of phosphate cathode. Herein, superstructure engineering is employed to increase the ionic characteristics of Fe─O bonds and the working voltage of Fe 2+ /Fe 3 redox couples. The combined analysis of advanced structural characterization and theoretical calculation indicates that the Fe 3+ ions can migrate to Na + vacancies to generate Fe/Na_v superstructure ordering by manipulating calcination temperature during synthesis. The Fe delocalization and electronic structure rearrangement can enlarge the energy gap between antibonding orbital and the Fermi energy level. As a concept proof, the as‐prepared Na 3 VFe(PO 4 ) 3 cathode with Fe/Na_v superlattice structure enables an increase in Fe 2+ /Fe 3 redox couples from 2.37 to 2.82 V, accompanied by the energy density increase from 325 to 350 W h kg −1 , compared with the conventional Na 3 VFe(PO 4 ) 3 electrode. This work paves the way for increasing the working voltage and energy density of NASICON type iron‐based phosphate cathodes for SIBs.