Understanding the effect of vinylene carbonate on SEI formation, morphology and stability on lithium metal anodes

Lithium metal batteries are promising for applications requiring high energy density storages, but their practical implementation is hindered by the high reactivity of metallic lithium with liquid electrolytes. Film-forming additives can contribute to stabilize the lithium/electrolyte interface by promoting a well-performing solid-electrolyte interphase (SEI). However, their specific influence on SEI formation mechanisms remains poorly understood, causing challenges in designing advantageous SEIs. In this work we reveal the role of the widely used electrolyte additive vinylene carbonate (VC) in SEI formation on lithium metal. An ab initio informed kinetic Monte Carlo approach is applied to bridge the gap between atomistic simulations and the timescale of seconds, enabling the first mechanistic investigation of VC-driven SEI growth beyond elementary reaction steps. We find that VC only significantly impacts SEI formation beyond the millisecond range with only a minor impact on the previous formation of the inorganic passivation layer close to the anode surface. Yet, it enables a chemically-driven polymerization process, passivating the surface without ongoing consumption of lithium. The resulting polymeric layer prevents dissolution of SEI and intermediate species and stops the continuous consumption of electrolyte species. Higher VC concentrations do not significantly alter the protective mechanism but result in a higher proportion of remaining additive, allowing for repassivation during operation. Overall, this study provides an in-depth molecular understanding of the influence of VC on SEI formation, paving the way to model-assisted SEI design.