Epitaxial growth of hyperbranched Cu/Cu₂O/CuO core-shell nanowire heterostructures for lithium-ion batteries

The careful design of nano-architectures and smart hybridization of expected active materials can lead to more advanced properties. Here we have engineered a novel hierarchical branching Cu/Cu₂O/CuO heteronanostructure by combining a facile hydrothermal method and subsequent controlled oxidation process. The fine structure and epitaxial relationship between the branches and backbone are investigated by high-resolution transmission electron microscopy. Moreover, the evolution of the branch growth has also been observed during the gradual oxidation of the Cu nanowire surface. The experimental results suggest that the surface oxidation needs to be performed via a two-step exposure process to varying humidity in order to achieve optimized formation of a core-shell structured branching architecture. Finally, a proof-of-concept of the function of such a hierarchical framework as the anode material in lithium-ion batteries is demonstrated. The branching core-shell heterostructure improves battery performance by several means: (i) The epitaxially grown branches provide a high surface area for enhanced electrolyte accessibility and high resistance to volume change induced by Li+ intercalation/extraction; (ii) the core-shell structure with its well-defined heterojunction increases the contact area which facilitates effective charge transport during lithiation; (iii) the copper core acts as a current collector as well as providing structural reinforcement.