Toward advanced sodium-ion batteries : A wheel-inspired yolk-shell design for large-volume-change anode materials

Yolk-shell structures have found great potential in addressing the huge volume change of alloy-type anodes for lithium-/sodium-ion batteries (LIBs/SIBs). The main challenges associated with yolk-shell structures are the sluggish electron/ion transfer at the yolk-shell interface caused by the point-to-point contact between the yolk and the shell, and the rupture of self-supporting carbon shells during a long-term cycle. Here, inspired by the structure of a wheel, we designed and fabricated a novel yolk-shell structure with a multipoint contact between the yolk and the shell (wheel-shell structure) through a general and scalable approach. The multipoint contact is achieved by bridging SnO₂ yolks and graphene shells using carbon nanoribbons, which allows a high-efficiency transfer of electrons and ions inside/outside the wheel-shell structure. Moreover, the interconnected graphene shells function not only as an electrical highway so that all active materials are electrochemically active, but also as a mechanical backbone to maintain the structural integrity. As an anode for SIBs, the wheel-shell structure exhibits extraordinary rate capability (153.3 mA h g^-1 at 10.0 A g^-1) and robust cycling stability (248.2 mA h g^-1 remaining after 1000 cycles at 1.0 A g^-1 with a capacity retention of 86.9%). These results demonstrate the most efficient SnO₂-based anode ever reported for SIBs. More importantly, the proposed strategy opens up new avenues to boost the electrochemical performance of large-volume-change anode materials for advanced battery systems.