The Three-Component Problem: Deconvoluting Electrolyte Component Roles in Solvation Structure and Silicon Calendar Aging

In this work, we deconvolute the roles of LiPF6, ethyl methyl carbonate (EMC), and vinylene carbonate (VC) in the electrolyte solvation structure and silicon anode calendar aging. By varying the mole fractions of the electrolyte constituents and utilizing a voltage hold protocol, we extract the roles of each component in the silicon anode calendar lifetime. Interestingly, the LiPF6 concentration does not affect calendar aging, whereas increasing and decreasing the EMC and VC mole fractions, respectively, lowers the electrode surface passivity. The calendar lifetime results align with EMC coordination numbers derived from MD simulations, which indicate the inner solvation sheath's role in calendar aging mechanisms. X-ray photoelectron spectroscopy data show that the solid electrolyte interphase (SEI) species can be tuned through the electrolyte formulation and solvation structure. Li+-EMC coordination structures decompose to form insoluble alkanes, C-O species, and lithium carbonate (Li2CO3), while the anion coordination complex decomposes into LiF and LixPOyFz. Postaging, the surface lithium content of the SEI is partially replaced by C-O species from preferential decomposition of noncoordinated VC. However, the inorganic lithium species (LiF, LixPOyFz, and Li2CO3) remain constant, suggesting the dissolution of nonpolar organolithium species due to EMC's low dielectric constant and high coordination number. This triggers continuous electrolyte decomposition to reform the SEI, which lowers the silicon anode's calendar lifetime.