Insight into microstructural and phase transformations in electrochemical sodiation–desodiation of a bismuth particulate anode

The microstructural stability of active-material particles is critical for maintaining the integrity of battery electrodes necessary for achieving high electrochemical performance. The correlation between the electrochemistry and structural evolution for the sodiation and desodiation of a Bi particulate anode was investigated using a combination of in operando synchrotron transmission X-ray microscopy and X-ray diffraction. Unique features not previously seen in the sodiation or lithiation of other alloying anodes were revealed. Specifically, an unprecedented serious delay in the crystal structure and microstructure transformation from NaBi to Na3Bi occurred. Microstructural evolution before the transformation is characterized by uniform and mild particle expansion without crack formation, and afterward by an abrupt and extensive fracturing of the particles. We demonstrate that the delayed structural transformation provides an opportunity to cycle the Bi particulate anode to high capacity while being immune to particle fracture or pulverization. The resistance to cracks and fractures over a wide sodiation depth is consistent with a high resistance to brittle fracture of NaBi predicted in the literature. The results may indicate a new direction for engineering fracture-free alloying anode materials for both Na-ion and Li-ion battery applications.