(Invited) Ion Conduction in Glassy Electrolytes for Sodium Ion Batteries – an Atomistic Perspective
Authors: Dive Aniruddha Mukund , Banerjee Soumik 
Published: February 2020

Solid-state sodium ion battery is a promising technology for energy storage in electrical grids. However, the characteristic low ionic conductivity of solid-state electrolytes is a major challenge towards commercialization of solid-state sodium ion batteries. Sulfide glasses have relatively high ionic conductivity at room temperature and are therefore considered promising as potential electrolytes. However, the energy barrier for ion hops in these electrolytes is highly sensitive to the chemical composition, which provides a tremendous opportunity to tailor the ionic conductivity. In-situ characterization of Na+ ion transport through experiments is extremely challenging. Atomistic simulations offer an economical route for determining the local structure of these glasses as well as Na+ ion transport mechanisms. Sodium sulfide – silicon sulfide [xNa2S – (1-x)SiS2] based electrolytes with varying compositions was chosen as a model system and atomistic simulations of various flavors were employed to determine ion conduction mechanism and evaluate ionic conductivity, each method having distinct advantages. Experimental X-ray scattering data was used for the purpose of model validation.

Ab initio molecular dynamics (MD) simulations are able to accurately model the specific chemistry based on on-the-fly quantum mechanical calculations to determine interatomic interactions. Augmented with nudged elastic band analysis, these calculations can determine activation energies for ion hops. However, the maximum system size that can be realistically modeled using ab initio MD is limited to few hundred atoms and therefore cannot reliably calculate the bulk properties. On the other hand, classical MD simulations employ empirical forcefields to define pairwise interactions between constituent ions in these glasses. This method can simulate much larger systems comprising hundreds of thousands of atoms. However, the inability of these predefined forcefields to accurately capture the complex chemistry in sulfide glasses makes it extremely challenging to accurately model these glasses. Reactive force fields, such as ReaxFF, is based on a bond order potential in conjunction with charge equilibration that is based on extensive parameterization based on quantum mechanical calculations of structure and energies. ReaxFF forcefields are highly system specific. Classical MD simulations involving ReaxFF provide an effective route to accurately capture bond breakage and formation, while simulating considerably large systems (~ 100,000 atoms) and therefore accurately evaluates ionic conductivity.

Based on simulation data from each of the above methods, their merits and demerits are discussed as pertains to the model system of sulfide glasses. We determined the ionic conductivities and activation energies for sodium ion hops and related them to the structural units and hop distances. Overall, these simulations provide fundamental insights into ion conduction mechanisms as well as correlate ionic conductivity with glass structure and composition. The combined approach demonstrates excellent promise in identifying optimal glassy electrolytes for sodium ion batteries.