Investigations of the structure of Na2S + P2S5 glassy electrolytes and its impact on Na+ ionic conductivity through ab initio molecular dynamics

Abstract

Abundant sodium reserves make sodium ion batteries a promising technology for high energy density applications such as grid energy storage. Identifying solid electrolytes with superior room temperature Na+ ion conductivity is critical for designing safe and high energy density batteries with enhanced rate capabilities. We evaluated sodium thiophosphates [x Na2S + (100-x) P2S5], potential glassy solid electrolytes (GSEs), using ab initio molecular dynamics (MD) simulations. We investigated the change in the local structure of these glasses with Na2S content. We also calculated the relative fractions of different structural units within local structure for different compositions and compared them with those observed from FTIR and NMR spectroscopy. We investigated the plausible reasons for presence of polysulfides that act as Na+ ion trapping sites thereby reducing the Na+ ion conductivity. We report the maximum room temperature Na+ ion conductivity of ~10−5 S cm−1 for the x = 75 composition. Overall, our calculations provide theoretical insights on the role of polysulfides and different structural units on the ionic conductivity of GSEs aiding in the design of high ionic conductivity GSEs.