Ultrathin Bismuth Nanosheets for Stable Na-Ion Batteries: Clarification of Structure and Phase Transition by in Situ Observation

Bismuth has garnered tremendous interest for Na-ion batteries (NIBs) due to potentially high volumetric capacity. Yet, the bismuth upon sodiation/desodiation experiencing structure and phase transitions remains unclear, which sets a challenge for accessing nanotechnology and nanofabrication to achieve its applicability. Here, we use in situ transmission electron microscopy to disclose the structure and phase transitions of layered bismuth (few-layer bismuth nanosheets) during Na⁺ intercalation and alloying processes. Multistep phase transitions from Bi → NaBi → c-Na₃Bi (cubic) → h-Na₃Bi (hexagonal) are clearly identified, during which the Na⁺ migration from interlayer to in-plane evokes the structure transition from ABCABC stacking type of c-Na₃Bi to ABABAB stacking type of h-Na₃Bi. It is found that the metastable c-Na₃Bi devotes to buffer the dramatic structure changes from thermodynamic stable h-Na₃Bi, which unveils the origin of volume expansion for bismuth and has important consequences for 2D in-plane structure. As the lateral ductility can efficiently alleviate the in-plane mechanical strain caused by the Na⁺ migration, the few-layer bismuth nanosheet exhibits a potential cyclability for NIBs. Our findings will encourage more attention to bismuthene as a novel anode material for secondary batteries.