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.