Essential Role of Spinel MgFe₂O₄ Surfaces during Discharge

Spinel magnesium ferrite (MgFe₂O₄) is a prospective anode material in lithium ion battery (LIB) due to its large theoretical capacity. Here, we employed Density Functional Theory (DFT) to study the contribution from diverse facets of three spinel systems of MgFe₂O₄, normal-spinel, mixed-spinel and inverse-spinel, to the initial discharge behaviors. The mixed-spinel (1 0 0) surface terminated by MgFeO_x is found to be the most active among the diverse surfaces studied. It can provide the high capacity, the high voltage and facile Li⁺ transport during the initial discharge stage. The high performance is found to be associated with the high surface activity to capture Li⁺ ions, and the ability to accommodate a large amount of Li⁺ ions and facilitate the sequential smooth transport to subsurface. The DFT-estimated discharge voltages based on the mixed-spinel (1 0 0) surface terminated by MgFeO_x are much higher than those using the stoichiometric bulk models and fit well with the corresponding experimental measurement at the initial stage. Our results develop new design strategies for optimization of particle morphologies, enabling the enhancement in stability and discharge performance of ferrite materials.