Summary: The electrodeposition of lithium metal is a critical process in lithium metal batteries. The molecular mechanism of the electron transfer step of this process may seem simple, but is actually quite complicated, as metal deposition involves both ion and electron transfer at the interface. This work unravels the complexity of the electron transfer mechanism using transient voltammetry with ultramicroelectrodes. We find that a Marcus-based framework of electron transfer accurately describes lithium deposition, whereas the commonly assumed Butler-Volmer model of electrode kinetics poorly described lithium deposition. Beyond providing a molecular picture of lithium (and more generally, metal) depostion, this work provides a strategy for understanding how the electron transfer kinetics affect the morphology of electrodeposited lithium.