Exposed facet engineering design of graphene-SnO2 nanorods for ultrastable Li-ion batteries

Many routes are developed to prepare ultrastable electrode materials for Li-ion batteries (LIBs), whereas it is effective but still challenge to design promising ones through exposed facet engineering route. Here, graphene-SnO2 material is selected as a potential candidate due to its high theoretical capacity and conductivity. The ab initio density functional theory (DFT) calculations are firstly used here to predict the possible property of SnO2 materials with different exposed facets: (211) facet is found to have the relatively high surface energy and a lower energy barrier towards Li+ when compared with other planes. Then the graphene-SnO2 nanorods with highly exposed (211) facets (GSn-211) is fabricated by utilizing a crystalline-spacing-matching method. When tested as anode for LIBs, GSn-211 indeed shows a great electrochemical property: ultrastable cycling stability (695 mAh/g at 0.2 A/g after even 500 cycles) and an outstanding rate capability (530 mAh/g at 5 A/g). The Li-ions anisotropic transport and the atomic-scale ledged behaviors of GSn-211 along (211) are further visualized via an in situ TEM at atomic level, in which the Sn/O atoms-peeling-off through (211) facet is observed. This indicates (211) planes hold high active sites towards Li-ions insertion, leading to a fast Li+ storage. Combined with theoretical calculations, their atomistic mechanical properties during lithiation are explored: the decrease of bulk modulus means the GSn-211 is softened after the initial lithiated state leads to better structural stability. These findings provide a novel perspective for the advanced anode design for LIBs via the exposed facet engineering method.