We investigate the origins of phase transformation hysteresis in electrodes of Li-ion batteries, focusing on the alloying reaction of Li with Sb. Electrochemical measurements confirm that the reaction path followed during Li insertion into Sb electrodes differs from that followed upon subsequent Li extraction. Results from first-principles calculations and NMR measurements indicate that Li 3 Sb is capable of tolerating high Li-vacancy concentrations. An unusually high Li mobility in Li 3 Sb facilitates over potentials during charging, which leads to a substantially larger driving force for the nucleation of Sb compared to that of Li 2 Sb. The differences in nucleation driving forces arise from a lever effect that favors phases with large changes in Li concentration over phases that are closer in composition along the equilibrium path. These properties provide an explanation for the observed path hysteresis between charge and discharge in the Li–Sb system and likely also play a role in intercalation compounds and other alloying reactions exhibiting similar phase transformation hysteresis.