Morphology-dependent Li+ ion dynamics in X-ray amorphous and crystalline Li3PS4 prepared by solvent-assisted synthesis

Abstract

Solid-state electrolytes with high ionic conductivity will be crucial for future energy storage systems. Among many possible materials, thiophosphates offer both favourable mechanical properties and fast ionic transport. β-Li₃PS₄, as a member of the thiophosphate family, has gained recent attention, due to its remarkable increase in Li⁺ ionic conductivity when prepared via solvent-assisted synthesis. Despite earlier studies, the lithium ion migration processes causing the increased conductivity remain, however, still uncertain. Here, we study both long-range cation transport and local Li⁺ jump processes by broadband impedance spectroscopy and nuclear magnetic resonance (NMR), respectively. In particular, we focus on the comparison between mechanochemical and solvent-assisted synthesis to determine the origin of the increased ionic conductivity observed in the latter. Our measurements reproduce the previously reported high ionic conductivity and reveal that synthesis conditions significantly affect the Arrhenius pre-exponential factor governing ionic conductivity. Diffusion-controlled ⁷Li (and ³¹P) NMR spin relaxation rates confirm rapid, anisotropic lithium ion hopping that is characterized by timescale-dependent activation energies E_a ranging from 0.40 eV (long-range transport, as also seen by conductivity spectroscopy) to values down to 0.09 eV (local barriers).