An objective in battery development for higher storage energy density is the design of compounds that can accommodate maximum changes in ion concentration over useful electrochemical windows. Not surprisingly, many storage compounds undergo phase transitions in situ, including production of metastable phases. Unique to this environment is the frequent application of electrical over- and underpotentials, which are the electrical analogs to undercooling and superheating. Surprisingly, overpotential effects on phase stability and transformation mechanisms have not been studied in detail. Here we use synchrotron X-ray diffraction performed in situ during potentiostatic and galvanostatic cycling, combined with phase-field modeling, to reveal a remarkable dependence of phase transition pathway on overpotential in the model olivine Li1-xFePO4. For a sample of particle size ∼113 nm, at both low (e.g., 75 mV) overpotentials a crystal-to-crystal olivine transformation dominates, whereas at interm...