Prediction of New Phase and Electrochemical Properties of Li 2 S 2 for the Application of Li-S Batteries

The intermediate product Li 2 S 2 plays a pivotal role in the charge/discharge process of lithium–sulfur batteries. However, the structural configuration and relevant properties of Li 2 S 2 are unclear. In this work, by using ab initio calculations, we present results of novel phases, average open circuit voltages ( V oc s), and electronic properties of the stable Li 2 S 2 . Two new Li 2 S 2 phases are predicted: orthorhombic ( Cmca ) and orthorhombic ( Immm ) structures. The calculated V oc s of hexagonal ( P 6 3 / mmc ), orthorhombic ( Cmca ), and orthorhombic ( Immm ) are 3.91, 3.95, and 3.88 V, respectively. In particular, the calculated band gap of the Immm structure is about 0.225 eV, which is smaller than that of Li 2 S. The narrow band gap of Li 2 S 2 derives from the electronic lump between the Li s state and S 3p state for the orthorhombic structure. Therefore, the electronic properties of Li 2 S 2 are markedly influenced by the structural configuration.