The Hidden Side of Nanoporous β-Li3PS4 Solid Electrolyte

Abstract

Solid-state batteries are enjoying a regained interest owing to the discovery of sulfide-based solid electrolytes with ionic conductivities comparable to their liquid counterparts. Among them, the metastable β-Li3PS4 polymorph has attracted great attention given its high room temperature ionic conductivity (≈0.15 mS cm−1) when synthesized via a solvent-mediated route. However, the origin of such a high conductivity together with its structural interplay remains unclear. Herein, an in-depth study of the THF-synthesized nanoporous β-Li3PS4 is reported. Synchrotron X-ray diffraction, NMR, and Raman spectroscopy confirm the presence of β-Li3PS4 as the only crystalline phase and also reveal an additional amorphous phase and remnant THF. Moreover, a clear dependence of the ionic conductivity of the material on the relative content of its phases is found, the amorphous one being the most conductive. Lastly, high chemical reactivity is found for the nanoporous β-Li3PS4 and other phosphosulfide electrolytes toward Li metal, evidenced in the spontaneous ignition when mixed together and further investigated by XRD and XPS. This study, which unveils a hidden side of the THF-synthesized β-Li3PS4 and other sulfide solid electrolytes, provides a new insight to the battery community when selecting a proper electrolyte for practical all-solid state batteries.