Mechanistic Insight into the Relationship between Electrolyte Additives, Solid Electrolyte Interphase Formation, and Lithium Morphology in Lithium Metal Batteries

Directing the morphology of lithium metal deposits during electrodeposition is crucial to the development of safe, high energy density batteries with robust cycle life. Towards this end, mechanistic insight is imperative to understand the relationship between electrolytes, additives, cycling conditions, and the resulting lithium morphologies. In this work we used a standard carbonate-based electrolyte while systematically adding water (0-250 ppm, corresponding to ~0-500 ppm HF) in Li||Cu cells to study the links between electrolyte composition, initial solid electrolyte interphase (SEI) formation, and morphology of electroplated lithium metal. Under conditions in which the electrolyte contains several hundred ppm added HF and applied constant currents on the order of 0.5 mA/cm2, this system yields electrodeposited lithium metal with a highly monodispersed columnar morphology, whereas HF-free electrolytes result in lithium metal with a mossy morphology. We used electrochemical characterization to investigate the HF reduction process, X-ray photoelectron spectroscopy to characterize SEI chemistry, wide angle X-ray scattering to probe the nanostructure of the initial SEI and crystallographic texture of electrodeposited lithium metal, scanning electron microscopy to observe the resulting lithium metal morphology, and operando small angle X-ray scattering to understand cyclability of columnar lithium. Our systematic experimental approach enables insights to be drawn concerning the underlying mechanisms of columnar lithium formation. This morphology arises from an SEI layer comprising crystalline LiF deposits, formed through selective reduction of HF, embedded in an amorphous matrix of solvent reduction products. This interphase structure contains fast lithium ion diffusion pathways which lead to a high nucleation density and uniform growth of lithium metal deposits. The mechanism proposed herein will help to inform future electrolyte additive design and rational cycling protocols for lithium metal batteries. Figure 1