Crystalline vs. amorphous Li4PS4I: Impact of structure on ionic transport and performances in solid-state battery

All-solid-state batteries (ASSBs) are emerging as next-generation energy storage solutions due to their potential advantages, including enhanced safety, higher energy density, and broader operational temperature ranges. Among various solid electrolytes, amorphous and crystalline Li₄PS₄I, have attracted interest due to their predicted high conductivity, and high moisture-tolerance. However, experimental studies have reported a wide variation in conductivity values for Li₄PS₄I, ranging from 0.03 to 3.5 mS.cm⁻¹ at 298 K which are significantly lower than theoretical predictions. Herein, by employing a combination of X-ray diffraction (XRD), ³¹P magic-angle spinning nuclear magnetic resonance (³¹P MAS NMR), electrical impedance spectroscopy (EIS), we demonstrate that controlling the crystallinity of Li₄PS₄I plays a crucial role in its electrochemical performance. Pair distribution function (PDF) analysis reveals the differences in local atomic arrangements between amorphous and crystalline Li₄PS₄I. Additionally, the analysis indicates that mechanical milling alters the local environment of PS₄ tetrahedra and iodide anions, which may explain the discrepancy in conductivity. Furthermore, ASSBs incorporating amorphous-ceramic Li₄PS₄I in the cathode composite exhibit enhanced cycling stability compared to amorphous Li₄PS₄I. These findings underscore the potential of tuning crystallinity as an effective approach to optimize the ionic transport properties and cycling performance of ASSBs, paving the way for further advancements in solid electrolytes.