Essential Role of Spinel ZnFe₂O₄ Surfaces during Lithiation

Spinel zinc ferrite (ZnFe₂O₄) is a well-known anode material in lithium ion batteries (LIBs) because of its large theoretical capacity. However, the high potentials observed at the initial stage of lithiation cannot be captured using a model of Li⁺ intercalation into the stoichiometric ZnFe₂O₄ bulk. Here, using density functional theory, we report for the first time that the ZnFe₂O₄ surfaces are responsible for the measured initial potentials. Among the three identified stable surfaces, ZnFeO₂-terminated ZnFe₂O₄(1 1 0), O-terminated ZnFe₂O₄(1 1 1), and Zn-terminated ZnFe₂O₄(1 1 1), both (1 1 1) surfaces display higher lithiation potentials than the (1 1 0) surface, and the estimated potentials based on Zn-terminated (1 1 1) fit well with the experimental observations, whereas using the models based on ZnFe₂O₄(1 1 0) and previously ZnFe₂O₄ bulk, the estimated potentials are much lower. In terms of Li⁺ diffusion, the Zn-terminated ZnFe₂O₄(1 1 1) surface is the most active, where the energetically favorable saturation of Li⁺ on the surface is able to facilitate the process. Our results provide a new strategy for the design of LIB materials, via controlling the particle shape and the associated surface characteristics, thus enhancing the discharging performance.