In-situ TEM imaging of FeF2 nanoparticles for their morphological evolution upon electrochemical lithiation, revealing the rapid formation of sub-nm Fe particles on the surface, followed by gradual formation of larger (1-3 nm) nanoparticles within the domain of the FeF2 particles. The movie was recorded at a frequency of 2 frames per second, with the TEM operated in the annular dark-field (ADF)-STEM mode (z-contrast imaging). The original video was accelerated by 42 times.
A first of a kind electrochemical cell was custom-built for operation inside the transmission electron microscope (TEM), capable of tracking lithium reactions and subtle changes occurring within individual nanoparticles in battery electrodes by atomic-level imaging and spectroscopy. The device has been applied for in situ direct observation of the reaction of lithium with iron fluoride (FeF2) nanoparticles, a new type of cathode material that holds promise to double the capacity of today’s intercalation electrodes. Detailed real-time experimental observations, assisted by computational calculations, revealed that lithium ions sweep rapidly across the surface of the nanoparticles in a matter of seconds. The transformation then moves slowly through the bulk in a layer-by-layer process that splits the compounds into distinct regions. This study represents an important step forward in our understanding of electrochemical conversion reactions and the role of electron and ion transport in the formation and evolution of nano-structures in the electrodes. This new capability of performing in situ nano-electrochemistry measurements is widely applicable for studying a variety of battery systems, and may eventually help pave the way for developing longer-lasting, higher-energy density lithium-ion batteries.