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 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.