Atomistic Mechanisms of Phase Transition of Spinel Oxides during the Li Intercalation-to-Conversion Reaction by In Situ TEM

Spinel oxides exhibit both intercalation and conversion reaction mechanisms during (de)lithiation, thus exhibiting great potential for Li-ion battery electrodes. Atomic-scale understandings of phase transitions upon Li-ion insertion are critical for optimizing oxide electrodes' performance. Herein, we employed single-crystalline Co3O4 nanosheets as a model and captured the phase transition dynamics process via in-situ TEM. We identified each step of lattice change (e.g., distortion and restructuring) of the spinel structure upon Li insertion and the atomistic mechanism of the following two-phase conversion reaction through in-situ electron diffraction and imaging. We also visualized the spatial distribution of typical transient phases during lithiation. Corroborated by theoretical calculations, we elucidate a comprehensive picture of the phase transition from spinel Co3O4 to rocksalt LixCo3O4, followed by the formation of a Li2O lattice and metallic Co precipitation. These atomic-scale insights provide fundamental understandings of phase transitions during solid-state electrochemical reactions for Li storage devices.