Abstract
The structural, dynamic, and electrical properties of a prototypical amorphous electrolyte are examined using a variety of theoretical and experimental techniques. We focus on the xNa 2 S-(100-x)SiS 2 system, which represents a potential application for all-solid-state batteries. First-principles molecular dynamics simulations, x-ray scattering, and conductivity experiments are performed on two select compositions of this binary: Na 2 S-SiS 2 and 2Na 2 S-SiS 2 . The validation of the model structures, which reproduce with excellent accuracy the measured structure functions permits examining in detail the atomic-scale picture and the effect of alkali addition. Results indicate a complete depolymerization of the network, which is characterized using the Q n formalism, and at high Na content consists essentially of isolated SiS 4 4 tetrahedra typical of the short-range order of the corresponding crystalline compound. As in other modified chalcogenides, the simulation results reveal the presence of homopolar Si-Si and S-S defects, the latter being known to enhance ionic conductivity while the former produces isolated (Si 2 S 6 ) 6 anions at high Na content. The measured temperature behavior of diffusivity and conductivity suggests typical Arrhenius behavior measured by complex impedance spectroscopy and not fully confirmed theoretically given the reduced ab initio statistics, while measured ionic conduction at room temperature is found to be similar to that of the corresponding thiogermanates. We measured an ionic conductivity of 2.9 × 10 -5 -1 cm -1 at room temperature for the 2Na 2 S-SiS 2 glass.