Rationalization of Diversity in Spinel MgFe2O4 Surfaces

Spinel magnesium ferrite (MgFe2O4) is a prospective anode material for lithium ion batteries (LIBs). Here, using density functional theory the first systematic study of diverse MgFe2O4 surfaces is reported in three types of spinel structures: normal, mixed, inverse, which can have significant impact on the initial lithiation. The results show that the faceting and therefore the shape of MgFe2O4 crystals strongly depend on the bulk structures. Upon going from normal, mixed to inverse spinel, the energetically preferred facets of MgFe2O4 vary from the combination of (1 0 0) and (3 1 1), soly {1 0 0} to a combination of (1 0 0), (0 0 1), (1 1 1), and (3 1 1), respectively, depending on the distribution of Mg2+ in the spinel structure. However, there is one common descriptor to the stable surfaces among all three MgFe2O4 systems: high density of Mg2+ exposed to the surface. This study rationalizes the essential effect of the substitution of Zn2+ for Mg2+ in ferrites and provides new insights on how to control the shape of ferrite materials and thus tune the performances of LIBs.