Hollow CoS/Co 9 S 8 Prisms With Tunable Shell Thickness for Fast and Durable Sodium Storage

ABSTRACT Hollow nanostructures are widely adopted for energy storage as the void space can accommodate the possible volume change during cycling, but how the shell would regulate reaction kinetics and mechanical stability remains insufficiently understood. Herein, hollow CoS/Co 9 S 8 prisms with tunable shell thickness are prepared to clarify the shell thickness‐dependent sodium storage behavior. Three samples with shell thicknesses of 95, 65, and 45 nm are obtained, and with the help of finite element analysis, CoS/Co 9 S 8 ‐65 with a shell thickness of 65 nm shows the most favorable balance between kinetic properties and structural robustness. CoS/Co 9 S 8 ‐45 with the thinnest shell promotes Na + diffusion but suffers from severe stress accumulation and structural degradation, whereas CoS/Co 9 S 8 ‐95 with the thickest shell improves mechanical stability but limits Na + diffusion and reaction activity. Benefiting from the optimized shell thickness, CoS/Co 9 S 8 ‐65 delivers exceptional sodium storage performance, achieving a high reversible capacity (638 mAh g −1 after 500 cycles at 1 A g −1 ), superior rate performance (496 mAh g −1 at 20 A g −1 ), and stable cyclic performance (2000 cycles with 488 mAh g −1 at 5 A g −1 ). In addition, the CoS/Co 9 S 8 ‐65//Na 3 V 2 (PO 4 ) 3 F 3 (NVPF) full cell demonstrates favorable cycling stability, confirming its potential for practical sodium storage applications.