Unravelling the role of Li2S2 in lithium–sulfur batteries: A first principles study of its energetic and electronic properties

• Crystal structures of Li2S2 are proposed and tested via discharge voltage calculation.
• Crystalline Li2S2 is subject to spontaneous disproportionation.
• Li2S2 exists only in the nonequilibrium regime.
• Electronic structure of Li2S2 is calculated via density functional theory.

It is widely believed that Li2S2 is one of the intermediate products of lithium–sulfur (Li–S) batteries. However, contradicting proposals regarding the role and even the existence of Li2S2 in Li–S batteries persist. To address this, we have carried out first principles calculations of the energetic and electronic properties of Li2S2 based on the most promising structures found through an evolutionary algorithmic study. Our proposed Li2S2 structure(s) give a discharge voltage of 2.11 V for 2Li+ + 2e− + Li2S2 → 2Li2S, which matches the lower plateau of the experimental discharge profile of Li–S cells. Our results also show that Li2S2 is subject to spontaneous disproportionation. Together these results indicate that Li2S2 plays a key intermediate non-equilibrium role in the discharge of Li–S batteries.

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