Atomic-Scale Tracking of Beam-Induced Phase Transitions in MgCr_1.5Mn_0.5O₄

Transition metal (TM) oxides, particularly manganese oxide spinels, are promising cathode materials for next-generation rechargeable batteries, offering reversible intercalation of multivalent ions, such as Mg2+, at high voltages and energy storage capacities potentially higher than conventional Li-ion cathodes. However, repeated electrochemical cycling of these oxide cathodes with Mg2+ can lead to irreversible structural changes, causing capacity fade and voltage losses. In this study, we utilize in situ transmission electron microscopy (TEM), including aberration-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS), to examine the electronic and structural changes in MgCr1.5Mn0.5O4 under electron beam exposure. We find that electron beam irradiation induces Mn migration in both the bulk and surface regions of the nanocrystals, leading to the formation of a MnO phase on the crystal surface, with the dynamics of this formation captured at the atomic scale. These results demonstrate the potential for in situ TEM to capture the atomic-scale dynamics of cation migration during the structural changes previously observed in battery cathodes throughout electrochemical cycling. Our study identifies the causes of unwanted secondary phase formation in multivalent spinel oxide cathodes, attributing it to phase separation and TM-ion diffusion.