Unraveling the SEI Mystery of Cis/Trans-Butylene Carbonate

Abstract

A solid electrolyte interphase (SEI) film is a key to stable battery performance. Of particular importance in designing SEI is in its strong dependence on electrolyte solvent. The most representative example is the distinct difference between ethylene carbonate (EC) and propylene carbonate (PC); EC-based electrolytes have ability to form good SEI film on graphite anode surface, while PC-based electrolytes do not, in spite of their slight difference (only one additional methyl group) in a molecular level. A similar relationship to the phenomena called “EC-PC mystery” was found in other cyclic carbonates: trans-butylene carbonate (t-BC, reversible with SEI) and cis-butylene carbonate (c-BC, irreversible without SEI),1,2 where the difference in single molecule is even smaller found just in cis-trans geometries. To unveil the mystery, details of SEI film formation processes on the graphite anode were simulated by Red Moon (RM) method,3–5 with interface models and hypothetical essential elementary chemical reactions as shown in the figure. We found that the SEI film formed in c-BC-based electrolyte contains less number of dimerized products Li2(BC)2, a primary component of good SEI film, and can cause lower stability of the film. As an origin of the reduced dimerization in c-BC, we identified larger solvation energy of c-BC to the intermediate LiBC and its smaller diffusion constant, which largely diminishes the LiBC-LiBC collision frequency. More details on contribution of each elementary steps will be discussed in the poster.