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

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 Fe2+/Fe3+ 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 Fe2+/Fe3 redox couples. The combined analysis of advanced structural characterization and theoretical calculation indicates that the Fe3+ 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 Na3VFe(PO4)3 cathode with Fe/Na_v superlattice structure enables an increase in Fe2+/Fe3 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 Na3VFe(PO4)3 electrode. This work paves the way for increasing the working voltage and energy density of NASICON type iron-based phosphate cathodes for SIBs.