Lithium dynamics at grain boundaries of β-Li3PS4 solid electrolyte

Abstract

Lithium diffusivity at the grain boundaries of solid electrolytes (SEs) can strongly impact the final performance of all-solid-state Li ion batteries (SSLBs). β-Li₃PS₄ (β-LPS) is a promising SE due to its good formability and low processing cost, but its total Li ionic conductivity can vary by orders of magnitude depending on the synthesis and processing conditions. One of the possible sources for this variability, the GB contribution, is still poorly understood to date. In this study, we systematically investigate the Li ion transport in tilt and twist GBs as well as amorphous/crystal interfaces of β-LPS by performing large-scale molecular dynamics (MD) simulations with a highly accurate moment tensor interatomic potential (MTP). We find that the Li ion conductivities at the GBs and amorphous/crystal interfaces (10⁻⁴–10⁻³ S cm⁻¹) are 1–2 orders of magnitude higher than that in the bulk crystal (10⁻⁵ S cm⁻¹). The Li ion diffusivity at twist GBs and amorphous/crystal interfaces shows no correlation with the degree of diffusion isotropy, while tilt GBs exhibit a negative correlation. Using topological data analysis, the Li pathway network in twist GBs and amorphous/crystal interfaces comprises persisting large Li ring sub-networks (n_Li ≥ 5) that closely resemble those found in the bulk amorphous structure, whereas more smaller and short-lived Li ring sub-networks (n_Li ≤ 5) are detected in tilt GBs and the bulk crystal. The concentration of persisting large Li ring sub-networks in the GB and amorphous/crystal interfaces is directly proportional to the degree of Li site disordering which in turn correlates with GB conductivity. Our findings provide useful insights that can guide the optimization of conductivity not only in β-LPS but also in other sulfide-type solid electrolytes through possible GB engineering.