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

Abstract Spinel Co 3 O 4 , 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 Co 3 O 4 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 Co 3 O 4 anode and firstly unveiled the rate-dependent morphological and structural evolution process. It has been demonstrated that the structural evolution from pristine Co 3 O 4 phase to the formation of Co/Li 2 O composites during lithiation (bias = −3 V). Impressively, the Co/Li 2 O 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 Co 3 O 4 anode for lithium ion batteries.