The Systematic Refinement for the Phase Change and Conversion Reactions Arising from the Lithiation of Magnetite Nanocrystals

Nanostructured materials can exhibit phase change behavior that deviates from the macroscopic phase behavior. This is exemplified by the ambiguity for the equilibrium phases driving the first open-circuit voltage (OCV) plateau for the lithiation of Fe₃O₄ nanocrystals. Adding complexity, the relaxed state for Li_xFe₃O₄ is observed to be a function of electrochemical discharge rate. The phases occurring on the first OCV plateau for the lithiation of Fe₃O₄ nanocrystals have been investigated with density functional theory (DFT) through the evaluation of stable, or hypothesized metastable, reaction pathways. Hypotheses are evaluated through the systematic combined refinement with X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) measurements, neutron-diffraction measurements, and the measured OCV on samples lithiated to x = 2.0, 3.0, and 4.0 in Li_xFe₃O₄. In contrast to the Li–Fe–O bulk phase thermodynamic pathway, Fe⁰ is not observed as a product on the first OCV plateau for 10–45 nm nanocrystals. The phase most consistent with the systematic refinement is LiFe₃O₄, showing Li+Fe cation disorder. The observed equilibrium concentration for conversion to Fe⁰ occurs at x = 4.0. These definitive phase identifications rely heavily on the systematic combined refinement approach, which is broadly applicable to other nano- and mesoscaled systems that have suffered from difficult or crystallite-size-dependent phase identification.