Computational study of crystalline and glassy lithium thiophosphates: Structure, thermodynamic stability and transport properties

Abstract

We present a theoretical work of solid electrolytes within (Li2S)x(P2S5)1−x (x=0.67,0.70,0.75), systematically including S-deficient compositions towards Li4P2S6. Using ab-initio molecular dynamics simulations amorphous structures are generated and investigated with respect to their properties and atomistic processes. We show that all glasses are thermodynamically meta-stable compared to their crystalline counterparts. The stability is connected to the occurrence of structural units and decreases from P2S74− over PS43− to P2S64−. Calculated Li tracer diffusion coefficients demonstrate similar transport properties for all glasses independent of the underlying structural units. This is confirmed by structure analysis revealing that all structural units create very similar chemical environments for Li. In all glasses, Li diffusion was found to be highly correlated and concerted motion was observed. In glasses obtained by severe melt-quenching the formation of “unusual” structural units with partly low excess energies were formed. In case of local Li deficiency (e.g., close to electrode interfaces) intermolecular S−S bonds between neighboring structural units were found to serve as novel charge compensation mechanism. These S−S bonds and the “unusual” units tend to lower the electronic band gap and could eventually lead to mixed electronic–ionic conduction mechanisms that might initiate further degradation reactions.