Fe-Based Polyanionic Solid-Solution Phases as High-Power and Low-Temperature Cathodes for Sodium-Ion Batteries

Anions generally constitute the structural framework of crystalline solids, making their extensive substitution, particularly for the formation of anionic solid solutions in polyanionic compounds, highly challenging. Herein, we propose that anion-regulated polyanionic compounds can be formed between main-group S (group VIA) and transition-group Mo (group VIB) and, accordingly, report a new series of Fe-based polyanionic intercalation compounds, Fe 2 [(MoO 4 ) 1– x (SO 4 ) x ] 3 (0 ≤ x ≤ 1), for potential low-temperature and high-power sodium-ion batteries. In this series, two new anionic-type solid-solution regions are identified: monoclinic phase (0 ≤ x ≤ 0.3) and rhombohedral phase (0.8 ≤ x ≤ 1). Sulfur substitution elevates the operating voltage by 0.22 V via a stronger inductive effect and improves Na + intercalation kinetics, whereas excessive sulfur renders pronounced structural vo…