Morphology-controlled synthesis of hematite hierarchical structures and their lithium storage performances

A series of hematite hierarchical structures, including amorphous hierarchical spheres with varied internal polycrystalline structures, single-crystalline nanoflakes and co-aligned micro-layered structures, have been fabricated by simply controlling the reaction time in a one-step hydrothermal procedure. In the early stages of the reaction, an Ostwald ripening process dominates the successive occurrence of single-shell, double-shell and yolk–shell spheres, which reoccur in reverse order under the effect of an H+ etching process on prolonging the reaction time. Afterwards, the oriented attachment mechanism takes charge of the formation of single-crystalline nanoflakes and co-aligned micro-layered structures at the expense of the previously formed spheres. It is found that the different adsorption configuration of the hydroxyl groups in different crystallographic planes is of fundamental importance to the morphology evolution from nanoflakes to micro-layered structures. The electrochemical measurements of these hierarchical structures show that their lithium storage properties are closely related to their crystallinity and morphology. The amorphous-based architectures exhibit quite similar electrochemical properties regardless of their morphologies, whereas the crystallized architectures present morphology-dependent electrochemical properties. Also, well-crystallized electrodes facilitate the access of lithium-ions and thus possess superior lithium storage performances, whether in the initial charge–discharge cycling or in the subsequent cyclic capacity retention.