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.

The precise mechanical property evaluation of a solid electrolyte interface (SEI) is crucial for the mechanical stability of the SEI on silicon anodes, which significantly expand during lithiation. Herein, cyclic loading tests and numerical methods were used to quantify the elastoplasticity and viscoelasticity of SEIs and the effects of the stress field in a silicon anode material below the SEI to precisely evaluate the elastic modulus of the SEI. Instrumented nanoindentation was used to quantitatively observe the indentation force response on the heterogeneous surface of composite electrodes. The mechanical properties of the SEI with a nanothin film structure probed by a simple analytical model assuming perfect elastic and homogeneous mechanical properties in a domain subjected to the indentation force resulted in significant elastic modulus overestimation. The substrate effect was significant, especially for reduced SEI thickness, and could cause an "apparently" mechanically bilayered structure of the SEI with a hard inorganic inner layer and a soft outer polymeric layer. The elastic modulus of the SEI formed in fluoroethylene carbonate (FEC) electrolyte (3.7 GPa) agreed with that previously predicted from the mechanical properties of cross-linked polymers in the SEI formed from a FEC electrolyte and supported the recent solid-state NMR results reporting the existence of polymeric species on the interfacial region of the FEC-SEI and the silicon anode.

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