Mesoporous shell engineering of yolk-shell Fe3O4/carbon microspheres enables fast and durable sodium storage

Magnetite (Fe 3 O 4 ) is regarded as a high-capacity anode candidate for sodium-ion batteries; yet its practical application is largely impeded by severe volume fluctuations during conversion reactions and sluggish Na + transport kinetics. Here, we integrate a mesoporous carbon shell with an internal void in yolk-shell Fe 3 O 4 /carbon microspheres. This architecture combines conductive confinement and space for volume changes with accessible ion-transport pathways, enabling improved utilization of Fe 3 O 4 during repeated sodium storage. The electrode echibits a reversible capacity of 616.9 mAh g −1 at 0.05 A g −1 , with favorable rate performance; at 1 A g −1 , 90.2% of its capacity remains after 1,500 cycles. A pouch full cell paired with a Na 3 V 2 (PO 4 ) 3 -based cathode delivers 130.7 mAh, extending the evaluation beyond sodium-metal half-cells. These findings establish the performance benefits of mesoporous confinement within the Fe 3 O 4 system and support further assessment of this anode in sodium-ion batteries.