Intrinsic limitation of conductivity in depolymerized sodium-ion glassy networks

Abstract

The electric properties of a model fast-ion electrolyte glass (100-$x$)SiS${}_2$ – $x$Na${}_2$S are investigated by means of classical molecular dynamics simulations. These materials are promising candidates for battery applications and the conductivity is thought to be essentially driven by the concentration of Na charge carriers. We first set up a Buckingham-Coulomb type potential able to describe the atomic structure and experimental structure functions (structure factor) in an improved fashion with respect to previous reported force fields. A systematic investigation of properties with modifier content Na${}_2$S (50 %$\le x\le$80 %) leads to an unexpected result, that is, a near constant behavior of the conductivity $\sigma \left(x\right)$ with Na${}_2$S increase for various isotherms in the liquid state. The analysis indicates that unlike Li-based electrolytes, the diffusivity ratio between network (Si,S) and modifier (Na) species is reduced, of about 6–8, and differs substantially with the corresponding lithium counterpart for which the same ratio is about 100. This leads to a contribution to conductivity dominated by (Si,S) atoms for the sodium system. While the concentration of network species in the range 66 %$\le x\le$80 % decreases, no dramatic increase in Na diffusivity is obtained, and the emergence of molecular Na${}_2$S in the structure at large modifier content also induces profound structural changes. Unlike Lithium glassy electrolytes, the design of Na-based batteries must, therefore, take into account the contribution of the network species.