Li-ion transport kinetics of Li10GeP2S12 solid electrolyte and its response to isovalent cation substitutions: Density functional theory and machine-learning-assisted molecular dynamics study

Abstract

Although the superior mechanical properties and non-flammability of solid electrolytes substantially improve the safety of Li-ion batteries, these merits are mostly nullified by lower Li ionic conductivity than the organic counterpart. However, the discovery of lithium superionic conductors, such as Li10GeP2S12 (LGPS) with its ionic conductivity comparable to that of the liquid electrolytes used in current Li-ion batteries, marks a turning point as it opens the potential for fast-charging solid-state batteries. In this study, we explore the LGPS-based solid electrolytes with even higher ionic conductivity by adding cation dopants. Using density functional theory (DFT) calculations and machine-learning-based molecular dynamics (ML-based MD) simulations, we demonstrate the crystal structures of LGPS after doping eight different cations of C, Co, Pb, Ti, Sb, Nb, Mn, and Bi. The thorough comparative analysis suggests Pb is a promising dopant that can enhance the ionic conductivity up to 43.5 %, with relatively less degree of synthesis difficulty. The intrinsic atomic mechanisms responsible for this enhanced ionic conductivity are further elucidated by analyzing two Li-ion transport mechanisms of paddle-wheel dynamics and cooperative hopping. With a particular focus on the dopant properties on the dynamics of Li-ions and surrounding tetrahedra, we also provide a fundamental guideline for developing solid electrolytes with superior conductivity.