Unlocking Limited Na‐Ion Transport Channel in Na ₂ Fe ₂ (SO ₄ ) ₃ for Ultrafast Na‐Ion Transport

ABSTRACT Alluaudite‐type Na 2 Fe 2 (SO 4 ) 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 Na 2 ─O/Na 3 ─O bonds to widen the sodium ion transport channels by Ca, Mn, Cu co‐doping. Specifically, Ca doping induced the elongation of the Na 2 ─O 5 and Na 2 ─O 5 ’ bonds; Mn doping resulted in the extension of the Na 2 ─O 1 bond; and Cu doping led to the increase in the Na 3 ─O 4 and Na 3 ─O 4 ’ bond lengths. Strikingly, the optimized Na 2 Fe 1.9 Ca 0.03 Mn 0.035 Cu 0.035 (SO 4 ) 3 exhibits ultrafast sodium ion diffusion coefficient in the range of 10 −10 to 10 −8 cm 2 ·s −1 , which is the highest one among the Na 2 Fe 2 (SO 4 ) 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 Na 2 ─O/Na 3 ─O bonds by multiple metallic cations substitution of Fe‐site in Na 2 Fe 2 (SO 4 ) 3 to realize high sodium ion transport kinetics.