Mechanism of Na-Ion Storage in BiOCl Anode and the Sodium-Ion Battery Formation

We systematically unravel the mechanism by which sodium ion reacts electrochemically with ionically layered BiOCl nanosheets. Solution-processed BiOCl nanosheets were cycled using slow scan cyclic voltammetry (50 mu V s(-1)) to reach the desired reaction voltages. Characterizations using in situ impedance spectroscopy and ex situ X-ray diffraction, Raman spectroscopy, and transmission electron microscopy are used to map the mechanism of Na-ion insertion and deinsertion in BiOCl nanosheets. It was found that BiOCl initially undergoes a conversion reaction to form metallic Bi. The metallic Bi further alloys with sodium ion to form Na3Bi and NaBi, a compound whose formation has not been reported before. We also detect the formation of BiO, Na3BiO4, and NaBiO3. Finally, BiOCl is used as anode against a Prussian blue cathode to prepare a full cell that is capable of providing an average discharge potential of similar to 2.2 V at the 100th cycle. The overall study reveals new insights and key differences in the mechanism of sodium-based electrochemical energy storage systems.