The capacity of most battery insertion reactions is limited by the number of open lithium sites in electrode’s crystal structure, usually reaching one per metal atom. Conversion reactions, where the metal oxygen bond is broken to form clusters of Li₂O and metal particles, can achieve far higher lithium stoichiometry. However, this process is much less reversible than intercalation and occurs at significantly lower potentials than theoretically expected, which has limited their commercial appeal. To pinpoint the underlying connection between structural changes and the observed overpotentials for lithiation, we have studied conversion reactions in MO_x (M = Ni, Fe, and Cr) thin films using the sub-nm interfacial sensitivity of in situ x-ray reflectivity. These studies show that conversion begins at elevated potentials (near 2 V) at the surface before propagating into the bulk. Heterostructures incorporating metal interlayers can also help define the direction and extent of the phase separation. We will discuss the possible origins of this effect and strategies to improve the kinetics and scalability of these model electrodes. This work was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.