Modeling Na2s Based Glassy Electrolytes for High Performance Solid State Sodium Ion Batteries

Abstract

Limited availability of lithium and safety concerns due to flammability of liquid electrolytes pose significant challenges to commercial lithium-ion batteries. To overcome these limitations, it is essential to explore technologies beyond conventional lithium ion batteries, particularly for electric power grid applications. Sodium ion batteries, particularly those employing solid electrolytes, are being explored as potential candidates. Solid state electrolytes are typically non – toxic and more environmental friendly, which provides them an edge over their liquid counterparts. However, obtaining relatively high ionic conductivity at room temperatures is challenging. In this work, we studied sodium sulfide (Na2S) based glasses as electrolytes for solid state sodium ion batteries. More specifically, sodium sulfide – silicon sulfide [0.5Na2S – 0.5SiS2] glass was explored. These glassy electrolytes were modeled using classical molecular dynamics (MD) approach. The interactions between various ions in the glass were defined using Buckingham type potential. All the glasses were formed using a typical melt – quench technique. Due to amorphous nature of the glass, a large number of initial glass structures were studied to obtain statistically reasonable model. The density of glass obtained through MD was observed to match quite well to the experimentally reported value. Further structural validation of the molecular model was performed by comparing the pair distribution function (PDF) for different pairs of ions for actual glass to those obtained from MD simulations. The ionic conductivity values for these glasses at room temperature have been observed to be in the range of ~ 10-5S/cm, which agreed well with the values obtained from the molecular simulations. Furthermore, the activation energy for ion hopping within the glass was obtained through MD and compared well with experimental data. A KMC site model was developed, based on inputs from MD, to evaluate the ionic conductivity of these glasses at a much larger length scale and the results agreed well with experimental measurements. Interactions at the anode – electrolyte interface play a key role in determining the performance of sodium ion batteries. Ab initio MD simulations of pure sodium metal and glassy electrolyte interface were performed. Structural configuration of the sodium anode – glass interface was estimated by calculating the density distribution and PDF’s for various ion pairs. Additionally, Na+ ion transport near the interface was studied carefully to determine specific metallic sites that accommodate Na+ ions. The distribution of these favorable sites along with their respective site energies were calculated and provided as inputs to KMC site model that simulated actual battery charging – discharging cycles based on the above inputs from ab initio MD.