In situ atomic‐scale observation of size‐dependent (de)potassiation and reversible phase transformation in tetragonal <scp>FeSe</scp> anodes

Abstract Potassium‐ion batteries (PIBs) are considered promising alternatives to lithium‐ion owing cost‐effective potassium resources and a suitable redox potential of −2.93 V (vs. −3.04 for Li + /Li). However, the exploration appropriate electrode materials with correct size reversibly accommodating large K ions presents significant challenge. In addition, reaction mechanisms origins enhanced performance remain elusive. Here, tetragonal FeSe nanoflakes different sizes designed serve as an anode PIBs, their live atomic‐scale potassiation/depotassiation revealed first time through in situ high‐resolution transmission electron microscopy. We found that undergoes two distinct structural evolutions, sequentially characterized by intercalation conversion reactions, initial behavior is size‐dependent. Apparent expansion induced observed small‐sized nanoflakes, whereas unexpected cracks formed along direction ionic diffusion large‐sized nanoflakes. The stress generation crack extension originating from combined effect mechanical electrochemical interactions elucidated geometric phase analysis finite‐element analysis. Despite behaviors, products Fe 2 Se after full potassiation can be converted back into original upon depotassiation. particular, exhibit better cycling well‐maintained integrity. This article successful demonstration visualization reveal size‐dependent dynamics. Moreover, it provides valuable guidelines optimizing dimensions advanced PIBs. image