Temporally and Spatially Resolved Visualization of Electrochemical Conversion: Monitoring Phase Distribution during Lithiation of Magnetite (Fe₃O₄) Electrodes

Fundamental understanding of transport properties across multiple size regimes is critical for the rational design of electrodes with conversion materials. While recent studies have effectively interrogated mass transport in the conversion material magnetite (Fe₃O₄), a remaining challenge is to understand how electron and ion transport progress in a thick electrode. To provide insight, this study performs characterization of Li/Fe₃O₄ electrochemical cells both in situ and operando using synchrotron energy dispersive energy diffraction (EDXRD). In situ EDXRD measurements performed after 14 days of open circuit voltage recovery exhibit phase homogeneity with no observable reaction front for rocksalt formation. Operando EDXRD results clearly reveal that the insertion and conversion reactions associated with lithiation of Fe₃O₄ initiate at the Li anode interface and propagate as a reaction front through the electrode. The variation between the in situ and operando measurements necessitated the development of an electrode scale model which described the Li⁺ transport barriers in the system and the redistribution of intermediate phases upon relaxation. The spatial redistribution of discharged phases was successfully described by a multiscale model which provided insight into the Li⁺ concentration gradient generated throughout the depth of the electrode during electrochemical testing. This work provides insight into critical agglomeration and lithium transport barriers that should be considered when developing high energy batteries with multiple electron transfer nanomaterials.