Reactions of graphene supported Co₃O₄ nanocubes with lithium and magnesium studied by in situ transmission electron microscopy

Reaction beyond intercalation and the utilization of metal ions beyond lithium-ions are two promising approaches for developing the next generation of high capacity and low cost energy storage materials. Here, we use graphene supported Co₃O₄ nanocubes and study their reaction with lithium, magnesium and aluminum using in situ transmission electron microscopy. On lithiation, the Co₃O₄ nanocubes decompose to Co metal nanoparticles (2 to 3 nm) and embed in as-formed Li₂O matrix; conversely, the CoO nanoparticles form on the Co site accompanying the decomposition of Li₂O in the delithiation process. The lithiation process is dominated by surface diffusion of Li⁺, and graphene sheets enhance the Li⁺ diffusion. However, upon charge with magnesium, the Mg²⁺ diffusion is sluggish, and there is no sign of conversion reaction between Mg and Co₃O₄ at room temperature. Instead, a thin film consisting of metal Mg nanoparticles is formed on the surface of graphene due to a process similar to metal plating. The Al³⁺ diffusion is even more sluggish and no reaction between Al and Co₃O₄ is observed. These findings provide insights to tackle the reaction mechanism of multivalent ions with electrode materials.