How does vinylene carbonate affect the formation of initial surface passivation layers on lithium metal? – A model-based analysis

Rechargeable batteries with pure lithium metal anodes are a promising alternative to graphite anodes, since they provide significantly higher energy densities [1]. Unfortunately, wide commercial application is still hindered by the surface instabilities between lithium metal and the liquid electrolyte. Ongoing side reactions between the anode and electrolyte components or lithium dendrite growth can cause persistent loss of active material and safety risks [2]. Hence, the design of a stable and well-passivating solid electrolyte interphase (SEI) is one of the major remaining challenges. Analogous to lithium-ion batteries, electrolyte additives can be used to tune the interphase properties. It is well-known from experiment that vinylene carbonate (VC) or fluoroethylene carbonate (FEC) can significantly improve the coulombic efficiency of lithium metal anodes [3]-[5]. However, unlike graphite anodes, SEI formation on lithium metal begins long before cycling, namely right after the first contact of the metal and the liquid electrolyte. In this regard, it was recently reported that VC already has a substantial impact on this primary SEI and in particular on its thickness [6].

However, very little is already known about how VC actually affects the initial SEI formation process, the initial SEI structure and composition as well as what the underlying molecular processes are. This information is difficult to access experimentally, since the initial SEI formation process is very fast and proceeds within timescales well below seconds. In contrast, theoretical approaches are well suited to obtain a mechanistic understanding of the initial SEI formation also on smaller timescales. Thereby, first principle approaches such as density functional theory (DFT) can be used to identify reaction pathways and energies. Based on this, mesoscale approaches such as kinetic Monte Carlo simulations can offer insights into the chemical composition and morphology of the SEI [7].

Therefore, in this work we use a 3-dimensional kinetic Monte Carlo model to analyze the effect of VC on the initial SEI formation on lithium metal within the first second after contact with a carbonate-based electrolyte. Thereby, the considered reactions were parameterized with a comprehensive DFT study on possible VC and EC polymerization pathways and their related-free-energy profiles [8]. We reveal the time-dependent changes in chemical composition and SEI morphology for different electrolyte compositions. We find a layered initial SEI with an inorganic inner and an organic outer layer. Furthermore, we observe that VC only has a minor effect on the SEI formation within the first microsecond, in which it only forms short oligomers with a chain length of two to three units. We further find, that the formation of longer polymer chains requires a first passivation of the lithium metal surface and rather proceeds on time scales of milliseconds and above. Overall, we provide detailed first-principle based insights on the initial SEI formation, as well as its structure and composition on lithium metal. Here, we focus on how VC plays its role as an electrolyte additive in lithium metal batteries.