Chemomechanical Simulation of LiF-Rich Solid–Electrolyte Interphase Formed from Fluoroethylene Carbonate on a Silicon Anode

Maintaining an electrochemically and mechanically stable solid–electrolyte interphase (SEI) is of fundamental importance to the performance of high-capacity anode materials such as silicon. In this study, a chemomechanical model was developed to analyze the stress and strain in a LiF-particle/polymer system. By chemomechanical simulations, the stress and strain developments in a LiF-rich SEI with an inorganic/organic nanocomposite structure, consisting of LiF particles and poly(fluoroethylene carbonate) formed on silicon, were investigated. The results revealed that the LiF particle distribution in the SEI has considerable influence on the stress development. The presence of the polymer at the LiF/Si interface significantly reduced the von Mises stress, while the direct bonding of LiF and silicon resulted in increased stress, which caused ductile fracture with fragile void formation. The lateral tensile stress and strain were particularly concentrated at the LiF/polymer interface, suggesting likely ductile fracture of the polymer in this domain. These findings also supported the results of recent in situ atomic force microscopy and time-of-flight secondary ion mass spectrometry studies, which proposed void formation in the SEI when the expansion of the underlying electrode applies tensile strains to the film, accompanied by the formation of Li-containing species inside the void structure, which are associated with capacity losses. The stress and strain concentrations increased with shorter interparticle distances and larger particles. The modeling results indicated that the richness of LiF particles (i.e., particle size or interparticle distance) must be optimized to maintain the stress at the polymer/particle interface within the fracture limit. More broadly, this study provides important guidelines for producing SEI layers that can simultaneously satisfy both electrochemical and mechanical criteria for the long-term passivation of silicon electrode surfaces.