In recent years there has been growing interest in the development of large-volume-change electrode materials for use in lithium-, sodium- and potassium-ion based systems to satisfy the demand for batteries with low cost and/or high energy density. Conversion and alloying battery materials are attractive options for such systems due to their high specific capacity. The conventional thought is that the larger volume changes due to reaction with larger ions (e.g., Na⁺ and K⁺) cause significant mechanical degradation. However, the mechanisms by which these ions react with high-capacity materials are largely unknown. In this work, in situ transmission electron microscopy (TEM) experiments are used to examine the nanoscale transformation mechanisms of two different high-capacity nanocrystal materials during reaction with various alkali ions. In both cases, we find that unexpected changes in morphology during reaction arise due to significant mechanical stress evolution within the nanocrystals. Overall, these results are important because they provide guidance for engineering high-capacity electrode materials with improved durability and cyclability.

In the first set of experiments, cubic FeS₂ nanocrystals were examined with in situ TEM as they underwent reaction with Li⁺, Na⁺, and K⁺. In all cases, the FeS₂ nanocrystals exhibited a conversion-type reaction, but mechanical fracture during reaction was only observed during lithiation (which caused the smallest volumetric changes). This non-intuitive behavior was found to be due to differently-shaped reaction fronts that formed during reaction with Li⁺ compared to Na⁺ and K⁺. Specifically, lithiation induced a reaction front with sharp corners, while sodiation and potassiation resulted in fronts with blunted corners. Chemomechanical modeling of the reaction-induced expansion showed that the sharp corners during lithiation led to higher tensile stress concentrations and thus particle fracture. These results indicate that previously-dismissed high-capacity materials may in fact exhibit good chemo-mechanical stability during reaction and cycling with Na⁺ and K⁺. Next, other in situ TEM experiments were performed to investigate reaction mechanisms of Sb nanocrystals. These experiments showed that Sb nanocrystals naturally evolve to form hollow structures during ion removal. This finding indicates that electrochemically optimized nanoscale structures can evolve naturally in certain alloying anode materials, which may obviate the need for complex and expensive synthesis of hollow nanostructures for maximizing cyclability and Coulombic efficiency in battery anodes.