Facet Dependence and Vacancy-Controlled Reversibility of the Spinel-to-Rocksalt Phase Transformation at Co₃O₄ Surfaces

Transition-metal spinel oxides have attracted considerable interest as high-performance electrodes for electrochemical energy storage and conversion, where irreversible or reversible spinel–rocksalt (S–R) phase transformation at the oxide surface has been frequently observed, which sensitively controls their performance. Exploring key factors controlling the S–R transformation and its reversibility at the atomic scale is important for understanding the electrochemical performance of spinel oxides. Using Co 3 O 4 nanoparticles as an example, which represent a promising electrocatalyst for the oxygen evolution reaction, we present in situ atomic-scale imaging of the S–R transformation by using aberration-corrected scanning transmission electron microscopy combined with the integrated differential phase contrast imaging technique to visualize oxygen anions and transition-metal cations simultaneously. We reveal that the S–R transformation is not only determined by the oxygen vacancy formation energy but also largely controlled by the surface polarity of the reconstructed rocksalt layer, leading to a faster S–R transformation at the (001) surface than the (111) surface. Moreover, cobalt and oxygen vacancies are directly identified at the spinel/rocksalt interface, leading to the formation of an intermediate defective rocksalt phase and a two-step S–R transformation process. The existence of the intermediate defective rocksalt phase or not decisively controls the reversibility of the two-step S–R transformation. The uncovered facet dependence and vacancy-controlled reversibility of the S–R transformation provide important insights into the shape-dependent reactivity and stability of spinel oxide electrodes for electrochemical energy applications.