Structural, dynamic, and diffusion properties of a Li6(PS4)SCl superionic conductor from molecular dynamics simulations; prediction of a dramatically improved conductor

Abstract

Li-Superionic-conducting materials are considered the most desirable solid electrolytes to enhance fast battery charging/discharging. Extensive experimental and computational studies have been reported on various Li-ion electrolytes but the detailed mechanism of long-range ion transport in solid state superionic conductors has not been elucidated. We report here the results of molecular dynamics simulations with applied electric fields to explain both cationic and anionic diffusion mechanisms leading to Li-superionic conductivity in Li₆(PS₄)SCl. We find that Li migration in this material occurs via a conjugated substitutional type diffusion involving rearrangements of three or more Li ions in a 3D matrix of the anions that are essentially stationary for temperatures below 350 K. In this mechanism the presence of an equal number of filled and empty sites plays an important role. The predicted Li-ion conductivity of 7.2 × 10⁻³ S cm⁻¹ for single phase Li₆(PS₄)SCl at 298 K agrees well with recent room temperature NMR and impedance measurements, 3.9 × 10⁻³ S cm⁻¹ and 7.1 × 10⁻³ S cm⁻¹, respectively. The calculated activation energy of 0.24 eV is comparable to 0.27–0.29 eV obtained from NMR. We predict that Li₅(PS₄)Cl₂ will have a conductivity over 10 times higher than that of Li₆(PS₄)SCl, making it the best superionic conductor.