Anomalous lithium ion migration in the solid electrolyte (Li2S)7(P2S5)3; fast ion transfer at short time intervals studied by PGSE NMR spectroscopy

Abstract

The sulfide-based solid electrolyte (Li2S)7(P2S5)3 has large ionic conductivity, and thus is an important candidate for application to solid-state lithium rechargeable batteries. In lithium solid conductors, a small lithium ion (Pauling ionic radius of 60 pm) migrates in space composed of standing anions, and such situations are not yet well understood experimentally as well as theoretically. Lithium ion migration for the crystalline (Li2S)7(P2S5)3 was observed over short observation times using the pulsed-gradient spin-echo (PGSE) 7Li nuclear magnetic resonance (NMR) techniques. Diffusive diffraction effects were apparent in the echo attenuation profiles indicating that motion of lithium ions was in some way constrained to a certain (i.e., ‘characteristic’) length scale. Prior to being diffracted, the lithium ions rapidly migrate similar to lithium ions in liquids. The diffractive behavior depends on observation time and the strength of the applied pulsed magnetic field gradient (PFG), which suggests that the diffusing lithium ions are distributed. The diffraction patterns give a longer characteristic distance for faster moving lithium ions. Diffusing lithium ions collide with obstacles such as standing anions many times at longer time intervals, which consequently results in slow migration. We believe that our findings will contribute to understand conduction mechanisms in solid conductors.