Rate-dependent electrochemical reaction mechanism of spinel metal oxide anode studied by in situ TEM

Highlights: • Lithiation and delithiation of Co₃O₄ nanoparticles were studied via in-situ TEM. • It demonstrated the rate-dependent morphological and structural evolution process for Co₃O_4. • Co₃O₄ exhibited a severe volume aggregation (6 nm) at high C-rate. Spinel Co₃O₄, as conversion-type transitional metal oxide, has attracted extensive interest for LIBs anode due to its high theoretical capacity. The application of transitional metal oxide including Co₃O₄ anode has been proved challenging due to voltage hysteresis, bad rate capability, extremely low initial Coulombic efficiency (typically 65∼70%), and volume change induced mechanical failure. Among the facing problems, the fundamental understanding of severe capacity fading at high C-rate is clearly needed to be unveiled. In this paper, we utilized in situ transmission electron microscopy (TEM) to study the electrochemical reaction process of spinel Co₃O₄ anode and firstly unveiled the rate-dependent morphological and structural evolution process. It has been demonstrated that the structural evolution from pristine Co₃O₄ phase to the formation of Co/Li₂O composites during lithiation (bias = −3 V). Impressively, the Co/Li₂O composites could fully delithiated into pure tightly-contacted CoO crystals (∼1.45 nm) at lower delithiated bias (+3 V); while the lithiated products partially transformed into the CoO crystals, and further suffered a severe aggregation (∼6 nm) at higher delithiated bias (+8 V). These findings advance the understanding of the mechanism of capacity fading at high current density (C-rate) in conversion-type spinel Co₃O₄ anode for lithium ion batteries.