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.