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The elastic moduli of polymeric components in a solid electrolyte interphase (SEI) critically affect its mechanical stability on Si anodes, which undergo large expansion during lithiation, in lithium-ion batteries (LIBs). However, the complex multicomponent structure of an SEI hinders direct measurement of the elastic moduli of its constituent polymer components. To address this, we proposed a theoretical methodology to determine the elastic modulus. First-principles calculations were performed to study the cross-linking structures and mechanical properties of the polymeric materials formed by the electrochemical reduction of common LIB electrolyte additives, fluoroethylene carbonate (FEC) and vinylene carbonate (VC). The energy barrier corresponding to the 1,2-radical shift in each polymer was used to evaluate its possible reaction mechanism and cross-linking sites. Mechanical analyses of the three-dimensional cross-linked polymers revealed that poly(FEC) and poly(VC) exhibited anisotropic Young's moduli (2.36−6.49 and 6.32−15.35 GPa, respectively). Our calculations confirm the elastic behavior of the polymeric species in the SEI formed by the reduction of VC and FEC on high-capacity Si anodes. The mechanical properties of polymers in the SEI identified herein can contribute to ongoing work on the chemomechanical analysis of SEIs with inorganic/ organic nanocomposite structures for achieving mechanically stable SEIs.

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