Hierarchical BiOCl flowerlike microspheres via a room-temperature solid-state chemical reaction as a new anode for rechargeable magnesium-ion batteries

Rechargeable magnesium-ion batteries (MIBs) have received much attention in recent years, but their development remains limited due to a lack of anode materials with high capacity and fast diffusion kinetics. Herein, for the first time, hierarchical BiOX (X = Cl, Br, I) flowerlike microspheres composed of interleaved nanosheets are constructed via a simple room-temperature solid-state chemical reaction as the anode for MIBs. Among them, BiOCl flowerlike microspheres deliver good cycling stability (110 mA h g-1 after 100 cycles) and a superior rate capacity (134 mA h g-1 at 500 mA g-1). This is attributed to their unique flowerlike microsphere structure that not only accommodates a volume change to maintain their structural integrity but also shortens the ion-transport path to improve the diffusion rate. Importantly, ex situ tests were carried out to clarify the phase and structure evolution of the BiOCl flowerlike microspheres during cycling. The results show that BiOCl is first transformed to Bi and then alloyed to Mg3Bi2 in the discharging process, and Mg3Bi2 is turned back to Bi in the charging process. Besides, the initial microsphere structure is essentially maintained during the discharging/charging process, indicating the better stability of the structure. The current study demonstrates that the structural design of flowerlike microspheres is an effective strategy to develop promising anode materials for MIBs.