Tailoring the void size of iron oxide@carbon yolk-shell structure for optimized lithium storage

High-capacity lithium-ion battery anode materials, such as transition metal oxides, Sn and Si, suffer from large volume expansion during lithiation, which causes capacity decay. Introducing sufficient void space to accommodate the volume change is essential to achieve prolonged cycling stability. However, excessive void space may significantly compromise the volumetric energy density. Herein, a method to control the void size in iron oxide@carbon (FeO _x@C) yolk-shell structures is developed and the relationship between the void space and electrochemical performance is demonstrated. With an optimized void size, the FeO_x@C yolk-shell structure exhibits the best cycling performance. A high reversible capacity of ≈810 mA h g ^-1 is obtained at 0.2 C, maintaining 790 mA h g^-1 after 100 cycles. This contrasts with FeO_x@C materials having either smaller or larger void sizes, in which significant capacity fading is observed during cycling. This contribution provides an effective approach to alleviate the volume expansion problem, which can be generally applied to other anode materials to improve their performance in LIBs.