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, Fe₂[(MoO₄)_1-<i>x</i>(SO₄)_<i>x</i>]₃ (0 ≤ <i>x</i> ≤ 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 ≤ <i>x</i> ≤ 0.3) and rhombohedral phase (0.8 ≤ <i>x</i> ≤ 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 volume changes during de/sodiation and limited electrochemical reversibility. The optimized monoclinic phase (FMSO) delivers superior Na storage capability (1.99 Na⁺ per formula) compared with the end-members (1.77 Na⁺ for Fe₂(MoO₄)₃ and 0.46 Na⁺ for Fe₂(SO₄)₃), while maintaining outstanding power capability even at low temperature (-40 °C). Structural evolution, electrochemical properties, and reaction mechanisms have been comparatively elucidated for this series of solid solutions. Furthermore, the concept is extended to rhombohedral Fe₂[(WO₄)_ξ(SO₄)_1-ξ]₃ (0 ≤ ξ ≤ 0.2) phases, suggesting a general rule for anionic solid-solution formation. These results broaden the chemical diversity of Fe-based redox, which typically needs to be framed in polyanionic crystalline for high-voltage operations, and open new directions for designing high-power and cost-effective cathodes for sodium-ion batteries.