Li2S is the final product of lithiation of sulfur cathodes in lithium-sulfur (Li-S) batteries. In this work, we study formation and diffusion of defects in Li2S. It is found that for a wide range of voltages (referenced to metal Li) between 0.17 V and 2.01 V, positively charged interstitial Li (Li+) is the most favorable defect type with a fixed formation energy of 1.02 eV. The formation energy of negatively charged Li vacancy () is also constant, and it is only 0.13 eV higher than that of Li+. For a narrow range of voltages between 0.00 V and 0.17 V, the formation energy of neutral S vacancy is the lowest and it decreases with decreasing the cell voltage. The energy barrier for Li+ diffusion (0.45 eV), which takes place via an exchange mechanism, is 0.18 eV higher than that for (0.27 eV), which takes place via a single vacancy hopping. Considering formation energies and diffusion barriers, we find that ionic conductivity in Li2S is due to both Li+ and , but the latter mechanism being slightly more favorable.
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