Defect‐Engineered Bulk Conversion Anodes for Fast and Temperature‐Adaptive Na ⁺ Storage

Abstract Conversion‐based metal sulfides/selenides anodes are promising for sodium‐ion batteries (SIBs) owing to their high capacities and good electronic conductivity. However, their large volumetric variation during cycling often leads to rapid capacity decay, hindering commercial applications. Meanwhile, developing a low‐cost and efficient synthesis strategy to enhance sodium storage remains challenging. Herein, defect engineering is introduced in bulk Fe ₇ Se ₈ (bulk‐Fe ₇ Se _8‐x ) to fabricate a carbon‐free and defect‐rich anode. The optimized bulk‐Fe ₇ Se _8‐x realizes remarkable fast‐charging performance and good temperature adaptability, achieving excellent cyclic stability (384 mAh g ⁻¹ after 1300 cycles at 5 A g ⁻¹ and 25 °C, 170.6 mAh g ⁻¹ after 1000 cycles at 2 A g ⁻¹ and 0 °C, and 371.7 mAh g ⁻¹ after 500 cycles at 5 A g ⁻¹ and 40 °C) and ultrahigh rate capability (up to 40 A g ⁻¹ ), significantly surpassing reported Fe ₇ Se ₈ anodes. Reaction mechanism and reaction kinetics are elucidated through in/ex situ characterization, kinetics analysis, and DFT calculations. Furthermore, the exploration of full cells demonstrates their potential application of bulk‐Fe ₇ Se _8‐x in SIBs. The universal strategy is also successfully applied to synthesize the high‐performance, defect‐rich bulk‐Fe ₇ S _8‐x and bulk‐CoSe _2‐x , which provides an effective approach for other conversion‐based anode materials.