Li2S is the final product of lithiation of sulfur cathode in Li-S batteries. Mechanism of Li diffusion in Li2S has not been studied so far. Such a study can help us to understand the origin of (measured) lower capacity of sulfur cathodes in comparison with the theoretical capacity. In this presentation, I will discuss formation energies of a variety of possible defects and their diffusion mechanisms in Li2S. We found that the pathway and energy barrier of Li diffusion in Li2S is potential dependent. 

To our knowledge, such a result has not been reported so far in any other electrode materials. Our results show that Li(2-δ)S (Li-vacancy-rich) is the most favorable defective structure of Li2S for the voltage ranges between 0.93 V and 2.03 V (widely-used operating voltage range of Li-S batteries). In this case, the vacant sites are dominant defect sites and Li diffusion occurs via single vacancy with relatively low energy barrier of 270 meV. However, Li(2+δ)S (interstitial-Li-rich) may also exist as a metastable phase at high cycling rates (far from equilibrium). Considering the fact that the lithiation of S cathodes is an intraparticle two-phase (core/shell S/Li2S) growth process, we conclude that lithiation of metastable S/Li(2+δ)S particles may be strongly hindered because of the high energy barrier of 450 meV for Li diffusion (via the exchange mechanism) within the Li(2+δ)S shells.