Atomic Resolution Study of Reversible Conversion Reaction in Metal Oxide Electrodes for Lithium-Ion Battery

Electrode materials based on conversion reactions with lithium ions have shown much higher energy density than those based on intercalation reactions. Here, nanocubes of a typical metal oxide (Co₃O₄) were grown on few-layer graphene, and their electrochemical lithiation and delithiation were investigated at atomic resolution by in situ transmission electron microscopy to reveal the mechanism of the reversible conversion reaction. During lithiation, a lithium-inserted Co₃O₄ phase and a phase consisting of nanosized Co–Li–O clusters are identified as the intermediate products prior to the subsequent formation of Li₂O crystals. In delithiation, the reduced metal nanoparticles form a network and breakdown into even smaller clusters that act as catalysts to prompt reduction of Li₂O, and CoO nanoparticles are identified as the product of the deconversion reaction. Such direct real-space, real-time atomic-scale observations shed light on the phenomena and mechanisms in reaction-based electrochemical energy conversion and provide impetus for further development in electrochemical charge storage devices.