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 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.