Synergistic sodiation of cobalt oxide nanoparticles and conductive carbon nanotubes (CNTs) for sodium-ion batteries

Abstract

Replacing lithium with sodium-ion batteries for energy storage is of enormous interest, especially from practical and economic considerations. However, it has proved difficult to achieve competitive figures of merit for sodium-ion batteries due to the lack of a detailed understanding of the reaction mechanism(s). Herein, we report a sodium electrochemical conversion reaction with Co₃O₄ nanoparticles decorated on carbon nanotubes (Co₃O₄/CNTs) utilizing in situ TEM, down to the atomic-scale. We observe synergetic effects of the two nanoscale components, which provide insights into a new sodiation mechanism, facilitated by Na-diffusion along a CNT backbone and CNT–Co₃O₄ interfaces. A thin layer of amorphous low conductivity Na₂O forms on the CNT surfaces at the beginning of sodiation. The conversion reaction results in the formation of ultrafine metallic Co nanoparticles and polycrystalline Na₂O, and fast diffusion of the reaction products which might be due to the quick migration of Na₂O under an electron beam. In the desodiation process, the dissociation of Na₂O and formation of Co₃O₄ due to the de-conversion reaction are observed.