Importance of Fraction of Inorganic Components in Solid Electrolyte Interphase in Carbonate-Based Electrolytes Investigated by Molecular Simulations

The stability of the solid electrolyte interphase (SEI) is critical for the performance of lithium-ion batteries, particularly when silicon-based anode materials are used. In this work, the ratio of inorganic SEI components is identified as a key factor governing SEI stability and battery performance by molecular simulations within the investigated systems. Molecular dynamics simulations on single-component and composite SEI models reveal that inorganic SEI components exhibit significantly higher cohesive energy and mechanical strength than organic counterparts, while heterogeneous mixing can create mechanically vulnerable domains. Additionally, an implicit-solvent coarse-grained molecular dynamics framework enables direct simulation of SEI formation. Preferential formation of inorganic species by suppressing organic SEI species yields thinner, denser, and more homogeneous SEI layers, which probably reduces the subsequent electrolyte decomposition. These computational insights are further supported by experiments on pouch cells with silicon-based anode materials by varying the content of fluoroethylene carbonate (FEC). The cycle retention and consumed electrons at the beginning of life measured by ultrahigh-precision coulometry correlate with the “fraction” of lithium fluoride (LiF) formed on the anode surface measured by nuclear magnetic resonance rather than the absolute amount of LiF or content of FEC.