Study on the Electrochemical Reaction Mechanism of ZnFe(2)O(4) by In Situ Transmission Electron Microscopy

A family of mixed transition–metal oxides (MTMOs) has great potential for applications as anodes for lithium ion batteries (LIBs). However, the reaction mechanism of MTMOs anodes during lithiation/delithiation is remain unclear. Here, the lithiation/delithiation processes of ZnFe(2)O(4) nanoparticles are observed dynamically using in situ transmission electron microscopy (TEM). Our results suggest that during the first lithiation process the ZnFe(2)O(4) nanoparticles undergo a conversion process and generate a composite structure of 1–3 nm Fe and Zn nanograins within Li(2)O matrix. During the delithiation process, volume contraction and the conversion of Zn and Fe take place with the disappearance of Li(2)O, followed by the complete conversion to Fe(2)O(3) and ZnO not the original phase ZnFe(2)O(4). The following cycles are dominated by the full reversible phase conversion between Zn, Fe and ZnO, Fe(2)O(3). The Fe valence evolution during cycles evidenced by electron energy–loss spectroscopy (EELS) techniques also exhibit the reversible conversion between Fe and Fe(2)O(3) after the first lithiation, agreeing well with the in situ TEM results. Such in situ TEM observations provide valuable phenomenological insights into electrochemical reaction of MTMOs, which may help to optimize the composition of anode materials for further improved electrochemical performance.