context: Combining the insights derived from the systematic set of experiments detailed in this report along with complimentary results from literature, we have uncovered a new understanding of the mechanisms underlying the growth of columnar lithium metal. A schematic representation of the proposed mechanism linking electrolyte additive concentration, initial SEI formation, and the resulting growth of lithium columns is shown in Fig. 13. The initially pristine, oxide-free copper surface enables the electrocatalytic reduction of HF at ∼2 V vs. Li/Li+ which forms (111) textured LiF deposits a few nanometers in size on the surface. Two morphologically distinct yet functionally equivalent LiF deposit morphologies are possible – uniform yet discontinuous LiF particles decorating the copper surface or a continuous polycrystalline film – with the former being more probable in this system when considering the substrate roughness and LiF crystallite size. This HF reduction and LiF deposition process slows significantly when HF becomes depleted near the interface. Subsequently, as the potential drops below 1 V, solvent molecules are decomposed on the remaining unpassivated surfaces between and/or on top of LiF particles, forming organic reduction products and slowing further reduction of electrolyte species. While the precise morphology of the LiF particle layer could not be directly imaged, both candidate morphologies shown schematically in Fig. 13 would result in an SEI with similar properties and identical effects on lithium nucleation and growth. Interfaces between individual crystalline LiF particles and/or between LiF particles and the amorphous matrix of solvent reduction products in the initial SEI act as fast lithium-ion diffusion pathways. These pathways serve to homogenize the electronic and ionic properties near the copper surface, enabling a high nucleation density of lithium metal as the potential of the working electrode is brought below 0 V vs. Li/Li+ and electrodeposition of the active material begins via an instantaneous nucleation event. High lithium-ion diffusion within the SEI created by these interfaces, which allows for easier movement of lithium ions laterally across the electrode surface, along with the layer being very thin, allow for uniform, lateral growth of the lithium metal deposits until they bump into one another. After lateral growth is inhibited the deposits are restricted to growing vertically, normal to the working electrode, resulting in the highly monodisperse (110) textured columnar morphology observed. The diameter of these lithium columns can be controlled by varying the current density during electrodeposition (Fig. S8†). Indeed, a columnar morphology may be the intrinsically preferred growth mode of lithium metal. However, such growth may be inhibited by the SEI formed in conventional electrolytes without additives due to non-uniform lithium-ion diffusivity and the presence of “hot spots” where preferential deposition occurs, whereas the nanostructured SEI formed in electrolytes with added HF would allow columnar growth to proceed uninhibited.