Understanding of Structural, Dynamic and Ionic Diffusion Characteristics of Li6(PS4)SCl Superionic Conductor from Classical Molecular Dynamics

Abstract

Solid-state batteries are preferred over commercial Li-ion batteries, due to their higher thermochemical stability, higher safety, and longer cycle life. Successful applications of solid-state batteries largely depends on electrode-electrolyte interfacial stability and fast Li-ion transport in the electrolyte.1 Among different electrolyte candidates from polysulfide, oxide, argyrodite, perovskite families - Li6(PS4)SCl looks very promising due to its lithium superionic conductivity at room temperature and very wide electrochemical stability window (0−7 V versus Li/Li+).2 We report here our results on molecular dynamics simulations with applied electric fields (LAMMPS+UFF)8 to explain both cationic and anionic diffusion mechanisms leading to the Li-superionic conductivity of a Li6(PS4)SCl single crystal. Charges on ionic species were scaled from DDEC6 calculated charges and the Li-ion charge is comparable to that calculated for Li-ions in Li-P-S systems using ReaxFF. We find that Li migration occurs via conjugated substitutional type diffusion involving rearrangements of three or more Li-ions in a 3D matrix of anions that are essentially stationary around room temperature. Figure 1 shows mean-square displacements (MSDs) of the ions in Li6(PS4)SCl in our 20 ns MD simulation. The MSD of the Li-ions reaches the Fickian regime for Li-diffusion after 1 ns of the simulation.The predicted Li-ion diffusivity (4x10-12 m2/s) and conductivity for single phase Li6(PS4)SCl is 6.7 mS/cm is in good agreement with solid state NMR measurements of 5.7 mS/cm.4 Our calculated activation energy of 0.26 eV is within experimentally reported range. 0.27 – 0.29 eV, from NMR measurements (Figure 2).3–6 We further used the two phase thermodynamic (2PT) method7 to predict the melting point. Also, a detailed study was performed to examine the directional electric field effect of different field strengths on the Li-ion diffusion in Li6(PS4)SCl.Financial support of this research from the Centre of Hong Kong Quantum AI Lab Ltd established by the University of Hong Kong (the InnoHK initiative) is gratefully acknowledged.