Uncontrollable Li dendrite growth and poor cycling performance are primary barriers hindering the practical use of lithium metal electrodes, despite its high energy density. The solid electrolyte interphases (SEIs) on Li electrodes are critical to overcoming both the barriers. In this paper, we demonstrate experimentally in traditional carbonate electrolyte that the new Li2S formed in the inner part of the SEI layer by introducing electrolyte additive 1,2-propyleneglycol sulfite ( PS) is more beneficial for the stability of SEI compared with Li2O. One reason is that Li2S is mechanically more stable for preventing crack formation during SEI formation, based on volume mismatch calculations. The second reason is Li2S tends to trap fewer electrons on its fractured surface than Li2O with the aid of first-principles calculations, and as a result, Li2S can resist electron leakage through the SEI and prevent Li-electrolyte side reactions more efficiently than Li2O. (C) 2019 The Electrochemical Society.