Unlocking limited Na‐ion transport channel in Na2Fe2(SO4)3 for ultrafast Na‐ion transport

Abstract

Alluaudite-type Na2Fe2(SO4)3 has emerged as a promising cathode material for sodium-ion batteries (SIBs) owing to its high operating voltage (∼3.80 V vs. Na+/Na). Nevertheless, its practical rate performance is hindered by the sluggish Na+ transport. In this work, we realized the synergetic elongation of Na2─O/Na3─O bonds to widen the sodium ion transport channels by Ca, Mn, Cu co-doping. Specifically, Ca doping induced the elongation of the Na2─O5 and Na2─O5’ bonds; Mn doping resulted in the extension of the Na2─O1 bond; and Cu doping led to the increase in the Na3─O4 and Na3─O4’ bond lengths. Strikingly, the optimized Na2Fe1.9Ca0.03Mn0.035Cu0.035(SO4)3 exhibits ultrafast sodium ion diffusion coefficient in the range of 10−10 to 10−8 cm2·s−1, which is the highest one among the Na2Fe2(SO4)3 cathode up to date. Density functional theory (DFT) calculations confirm that co-doping can reduce the Na+ migration barrier. The sodium ion half-cell using this co-doped cathode delivers excellent rate capability (97, 80, and 69 mAh·g−1 at 0.1, 1.0, and 3.0 A·g−1, respectively) and excellent cycling stability of 5000 cycles. Our work provides new insights on the structural evolution of Na2─O/Na3─O bonds by multiple metallic cations substitution of Fe-site in Na2Fe2(SO4)3 to realize high sodium ion transport kinetics. Copyright © 2026 Wiley-VCH GmbH