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

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 Fe₇Se₈ (bulk-Fe₇Se_8-x) to fabricate a carbon-free and defect-rich anode. The optimized bulk-Fe₇Se_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 Fe₇Se₈ 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-Fe₇Se_8-x in SIBs. The universal strategy is also successfully applied to synthesize the high-performance, defect-rich bulk-Fe₇S_8-x and bulk-CoSe_2-x, which provides an effective approach for other conversion-based anode materials.