We report on a comparative density functional theory (DFT) study of ethylene carbonate (EC) decomposition on unlithiated, Li-covered, and fully-lithiated Sn surfaces as well as a pure Li surface. Results show that EC molecule does not dissociate on unlithiated Sn modelled by the clean β–Sn(100) surface, but rather adsorbs intactly via dispersion interaction. However, EC molecule spontaneously dissociates over Li-covered β–Sn(100), fully-lithiated Li17Sn4(001), and Li(100) surfaces. The dissociated EC molecule (C2H4+CO3) is energetically much more favorable than the intact EC molecule on these surfaces. We find that the dissociative adsorption of EC molecule is stronger on Li/β–Sn(100) and Li17Sn4(001) than on Li(100) in spite of larger electron transfer in the last case. This result indicates that besides electron transfer, chemical composition also plays an important role on dissociation of EC molecules. A detached CO 3 − 2 anion catches two Li atoms from the Li-rich surfaces and forms a Li2CO3 molecule, which is the building block of a main component of the solid electrolyte interphase (SEI) layer on Sn anodes according to the experimental observations.