context: In this work, a mechanism is proposed to explain the phenomenon of columnar lithium metal deposition. An electrolyte additive, such as HF, is selectively reduced at high potential vs. Li/Li+ to form uniformly distributed crystalline LiF particles with preferred crystallographic texture which are then encased in an amorphous matrix of solvent reduction products, as evidenced by systematic electrochemical analysis and in situ X-ray surface scattering. The LiF quantity and distribution are directly tunable by choosing an appropriate additive concentration and electrochemical cycling rate. Interfaces between LiF and the amorphous phase act as fast lithium-ion diffusion pathways, promoting a thin and more uniform SEI which leads to a very high lithium metal nucleation density relative to additive-free systems followed by nearly-isotropic and eventually vertical columnar growth, observed in real-time using operando small angle X-ray scattering. Understanding this process step-by-step provides new insights into the role of electrolyte additives and provides new information for the rational design of such additives. As HF is damaging to cathodes, current collectors, and other cell components, new additives which decompose at high potentials vs. lithium and direct the formation of a columnar morphology should be developed for practical use in lithium metal batteries.