Pressure effect on mechanical stability and ground state optoelectronic properties of Li2S2 produced by Lithium-Sulfur batteries discharge: GGA-PBE, GLLB-SC and mBJ investigation

Abstract

According to several previous works, Li2S2 occurs during discharge of lithium-sulfur (Li-S) batteries. Several information on the physical properties of this compound remain unknown including those of its ground state. In this work, a study of the pressure effect on the elastic and optoelectronic properties of Li2S2 in its tetragonal structure of P4(2)/mnm space group was carried out. According to the previous works and according to the structural results, which have been obtained by GGA-PBE and optPBE-vdW funtionals, we have found that P4(2)/mnm-Li2S2 energy is the lowest compared to other studied structures (P6(3)/mmc, P-62m and Immm), this can indicate that P4(2)/mnm space group structure may represent its ground state structure. The study of the pressure effect on the elastic properties has shown that P4(2)/mnm-Li2S2 maintains its mechanical stability for a pressure range between 0 and 2.5 GPa. The study of the directional dependence of Young's modulus has shown that it is anisotropic and therefore indicates the elastic anisotropy of the studied compound. According to the electronic results, the fundamental band-gap of P4(2)/mnm-Li2S2 is of an indirect nature, its values found by mBJ and GLLB-SC are close to each other. The pressure effect on the possible transitions between the valence band top and the conduction band bottom and on the variations of the refractive index and the absorption coefficient according to both optical directions has been studied by determining the dielectric function. The found optical results have shown that P4(2)/mnm-Li2S2 has negative uniaxial birefringence.