How Crystallite Size Controls the Reaction Path in Nonaqueous Metal Ion Batteries: The Example of Sodium Bismuth Alloying

Crystallite size effects can influence the performance of battery materials by making the structural chemistry deviate from what is predicted by the equilibrium phase diagram. The implications of this are profound: the properties of many battery materials should be reassessed. Sodium ion battery anodes made from nanocrystalline bismuth form different phases during electrochemical cycling compared to anodes with larger crystallites. This is due to the formation of a metastable cubic polymorph of Na3Bi on the crystallite surfaces. The structural differences (weaker Na-Bi bonds, different coordination of Na to Bi) between the metastable cubic Na3Bi phase found in the nanocrystals and the hexagonal equilibrium polymorph which dominates the larger crystallites offer an explanation for the improvements in cycling behavior observed for the nanostructured anode.