Nanoscopic Imaging the Lithiation of Sulfur Nanoparticles under Electron Beam Irradiation

In situ Transmission Electron Microscopy (TEM) provides powerful insights into the reaction mechanisms of Lithium-Sulfur (Li-S) batteries. However, distinguishing intrinsic electrochemical behaviors from artifacts induced by high-energy electron beam irradiation remains a critical challenge. Here, we systematically investigate the lithiation kinetics of sulfur nanoparticles triggered exclusively by electron beam irradiation, decoupling beam effects from electrochemical driving forces. We first conduct control experiments on pure lithium oxide (Li₂O) and pure sulfur to assess their stability under electron irradiation, and then monitor lithiation behavior in a mixed system of sulfur and lithium oxide (Li₂O), under varying irradiation times and temperatures. We report a striking "explosive" lithiation phenomenon, characterized by a massive volume expansion of up to 8300% and rapid kinetics (19312 nm² s^-1), which starkly contrasts with the ∼80% expansion observed in conventional electrochemical cycling. By conducting comparative experiments across a wide temperature range (25°C to -150°C), we identify the thermal effect of the electron beam as the dominant driving force; notably, the explosive reaction is completely suppressed at cryogenic temperatures (-150°C). Furthermore, we observe unique beam-induced artifacts, including directional cavity formation and rapid phase transitions from crystalline S to amorphous Li₂S. This work establishes a critical baseline for distinguishing beam-induced damage from genuine electrochemical reactions in in situ TEM studies and provides nanoscopic insights into the thermal runaway mechanisms of sulfur cathodes under high-energy abuse conditions, underpinning accurate characterization of Li-S battery materials and development of advanced battery systems.