Probing the Li Insertion Mechanism of ZnFe₂O₄ in Li-Ion Batteries: A Combined X-Ray Diffraction, Extended X-Ray Absorption Fine Structure, and Density Functional Theory Study

We report an extensive study on fundamental properties that determine the functional electrochemistry of ZnFe₂O₄ spinel (theoretical capacity of 1000 mAh/g). For the first time, the reduction mechanism is followed through a combination of in situ X-ray diffraction data, synchrotron based powder diffraction, and ex-situ extended X-ray absorption fine structure allowing complete visualization of reduction products irrespective of their crystallinity. The first 0.5 electron equivalents (ee) do not significantly change the starting crystal structure. Subsequent lithiation results in migration of Zn²⁺ ions from 8a tetrahedral sites into vacant 16c sites. Density functional theory shows that Li⁺ ions insert into 16c site initially and then 8a site with further lithiation. Fe metal is formed over the next eight ee of reduction with no evidence of concurrent Zn²⁺ reduction to Zn metal. Despite the expected formation of LiZn alloy from the electron count, we find no evidence for this phase under the tested conditions. Additionally, upon oxidation to 3 V, we observe an FeO phase with no evidence of Fe₂O₃. Electrochemistry data show higher electron equivalent transfer than can be accounted for solely based on ZnFe₂O₄ reduction indicating excess capacity ascribed to carbon reduction or surface electrolyte interphase formation.