Colloidal Bismuth Nanocrystals as a Model Anode Material for Rechargeable Mg-Ion Batteries: Atomistic and Mesoscale Insights

At present, the technical progress of secondary batteries employing metallic magnesium as the anode has been severely hindered due to the low oxidation stability of state-of-the-art Mg electrolytes, which cannot be to explore high-voltage (>3 V versus Mg²⁺/Mg) cathode materials. All known electrolytes based on oxidatively stable and salts, such as Mg(ClO₄)₂ and Mg bis(trifluoromethanesulfonimide), react with the metallic magnesium, forming a passivating layer at its surface and preventing the reversible plating and stripping of Mg. Therefore, in a-term effort to extend the upper voltage limit in the exploration of future candidate Mg-ion battery cathode materials, anodes have attracted considerable attention due to their efficient magnesiation and demagnesiation alloying in such electrolytes. In this context, we present colloidal Bi nanocrystals (NCs) as a model anode material for the of cathode materials for rechargeable Mg-ion batteries. Bi NCs demonstrate a stable capacity of 325 mAh g⁻¹ at least 150 cycles at a current density of 770 mA g⁻¹, which is among the most-stable performance of Mg-ion battery materials. First-principles crystal structure prediction methodologies and ex situ X-ray diffraction measurements that the magnesiation of Bi NCs leads to the simultaneous formation of the low-temperature trigonal structure, α-₃Bi₂, and the high-temperature cubic structure, β-Mg₃Bi₂, which sheds insight into the high stability of this reversible reaction. Furthermore, small-angle X-ray scattering measurements indicate that although the monodispersed, nature of the Bi NCs is indeed disturbed during the first discharge step, no notable morphological or structural occur in the following electrochemical cycles. The cost-effective and facile synthesis of colloidal Bi NCs and their high electrochemical stability upon magnesiation make them an excellent model anode material with which to progress in the field of Mg-ion secondary batteries.