Come for predictions, stay for complexity: Synthesis and experimental probing of ionic conductivity in Li9B19S33

Abstract

Lithium thioborates, despite their potential cost-effectiveness and low density, have received considerably less attention as solid electrolytes compared to their thiophosphate counterparts. A primary obstacle to their widespread investigation has been the inherent challenge in synthesizing single-phase materials. Computational studies have predicted several lithium thioborate phases exhibiting high ionic conductivity, with Li9B19S33 notably predicted to reach 80 mS/cm. However, experimental validation of these theoretical predictions remains absent. This work addresses this gap by detailing a successful synthesis of the previously elusive Li9B19S33 phase, facilitated by in situ temperature dependent powder X-ray diffraction (PXRD). Our findings reveal the peritectic nature of this phase formation, necessitating an excess of boron sulfide in the reaction mixture. We further present a comprehensive structural characterization of Li9B19S33 utilizing spectroscopic techniques like NMR, FT-IR, and diffuse reflectance and report on ionic conductivity of Li9B19S33 samples. Solid-state 6Li NMR line narrowing experiments revealed an ion mobility activation energy of 0.26 eV whereas activation energies derived from impedance spectroscopy measurements were significantly higher, resulting in lower than theoretically predicted ionic conductivity. Herein the successful synthesis of the Li9B19S33 phase offers a route to obtaining other challenging single-phase thioborates that can be used for all-solid-state batteries.