Na+ Ion Transport in Glassy Electrolytes for Solid-State Sodium Ion Batteries: A Reactive Molecular Dynamics Study

Abstract

All solid-state sodium ion battery is a promising technology for electrical grid energy storage. The low ionic conductivity of solid-state electrolytes is a major challenge towards commercialization of solid-state sodium ion batteries. Sulfide glasses are potential electrolytes possessing excellent ionic conductivity at room temperature for high performance sodium ion batteries. Understanding the Na+ ion transport in these glasses is critical towards developing novel electrolytes for these all solid-state sodium batteries. In-situ characterization of Na+ ion transport through experiments is extremely challenging. Molecular dynamics (MD) simulations offer an economical route for investigating the local structure of these glasses as well as Na+ ion transport mechanisms. Classical MD simulations employ empirical forcefields to define pairwise interactions between constituent ions in these glasses. These empirical forcefields captures the short-range order accurately, but cannot capture the medium-range order. Additionally, these empirical forcefields are not to account for the complex chemistry in sulfide glasses makes it extremely challenging to model these glasses. While ab initio MD simulations are able to overcome these limitations, the maximum system size that can be realistically modeled using ab initio MD is limited (400-500 atoms) and cannot account for the bulk properties of these glasses. Reactive force fields, such as ReaxFF, provide an effective route to accurately capture bond breakage and formation, while simulating considerably large systems (~ 100,000 atoms). These reactive force fields (ReaxFF) are based on bond-order formalism in conjunction with charge equilibration scheme. ReaxFF can more accurately capture the complex electrode-electrolyte interfacial reactions as compared to both classical and ab initio MD. Understanding these interfacial reactions is extremely crucial in developing a stable interface for high performance sodium ion batteries. ReaxFF forcefields are highly system specific and need to be parameterized for an individual system. In the present study, we employed Monte Carlo Force Fields (MCFF) optimizer to parameterize ReaxFF for x Na2S – (1-x) SiS2 glasses. We utilized ReaxFF for Li/Al/Si/O as a starting forcefield which was expanded for the x Na2S – (1-x) SiS2 glasses. Parameterization was performed by fitting against formation energies, atomic configurations and charge distributions for a range of representative clusters and equations of state obtained through DFT calculations. The parameterized reactive forcefields were validated by comparing the pair distribution functions (PDF’s) obtained through MD simulations to those obtained through X-ray scattering experiments. Na+ ion transport was investigated in these glasses characterizing the changes in the local structure with composition. Interfacial reactions between pure sodium metal and glassy electrolyte were further analyzed using these parameterized forcefields.