Sodium is an earth abundant element, which makes sodium ion batteries a promising technology for grid-scale energy storage applications. Due to safety concerns with liquid electrolytes, extensive research has been carried out over the past decade to develop new solid-state electrolytes. However, developing solid-state electrolytes with relatively high ionic conductivity at room temperatures is a major challenge. Sulfide glasses have been reported to possess excellent ionic conductivity making them potential electrolytes for high performance sodium ion batteries. With an aim to develop novel electrolyte for high performance sodium ion batteries, our present work focuses on analyzing the local structure of sulfide glasses and its subsequent impact on transport properties through atomistic simulations, in conjunction with experimental characterization. As model systems, we studied x Na₂S – (1-x) P₂S₅ glasses. The polarization and resonance effect of the P=S bonds in the glass complicates the parametrization of interatomic potentials necessary for classical MD simulations. Therefore, we employed ab initio MD simulations that perform quantum calculations on the fly while solving equations of motions at each MD step to accurately capture the complex chemistry.

As model systems for our calculations, we considered a range of electrolytes with composition x Na₂S – (1-x) P₂S₅ (x = 0, 0.33, 0.5, 0.6, 0.67, 0.75). The glasses were formed using melt-quench technique and the density of the glasses matched well (within 5%) with the experimentally measured value. We analyzed the local structure of these glasses by calculating the radial pair distribution functions (RDF’s) from ab initio MD and compared it to those obtained through X-ray scattering experiments to validate the model. The FTIR and NMR characterization of these glasses show presence of different structural units that effectively control the ionic conductivity of these glasses. We calculated the fractions of each of these functional units present in the glasses from ab initio MD. At 75% Na₂S, all the sulfur present in the structure is non-bridging due to presence of purely [PS₄]^3- groups in the glass. At concentration lower than 75% Na₂S, other structural units containing bridging sulfur such as [P₂S₇]^4- dominate the local structure. On the other hand, at concentrations above 75% Na₂S, additional structural units corresponding to [PS₃]^3- and polysulfides are observed along with [PS₄]^3- due to excess Na₂S content. Increase in concentration of polysulfides leads to lower ionic conductivity. As a result, optimal ionic conductivity of these glasses is observed at 75 Na₂S – 25 P₂S₅ composition. The calculated ionic conductivity of these glasses at room temperature was ~ 10^-6 S/cm, which matches well in terms of order of magnitude with experimentally measured values. A relatively high ionic conductivity (~ 10^-5 S/cm) can be achieved at slightly elevated temperatures of around 60 °C, making these sodium thiophosphate glasses promising electrolytes for solid-state sodium ion batteries.