Amorphous versus Crystalline Li₃PS₄: Local Structural Changes during Synthesis and Li Ion Mobility

Glass–ceramic solid electrolytes have been reported to exhibit high ionic conductivities. Their synthesis can be performed by crystallization of mechanically milled Li2S–P2S5 glasses. Herein, the amorphization process of Li2S–P2S5 (75:25) induced by ball milling was analyzed via X-ray diffraction (XRD), Raman spectroscopy, and 31P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. Several structural building blocks such as [P4S10], [P2S6]4–, [P2S7]4–, and [PS4]3– occur during this amorphization process. In addition, high-temperature XRD was used to study the crystallization process of the mechanically milled Li2S–P2S5 glass. Crystallization of phase-pure β-Li3PS4 was observed at temperatures up to 548 K. The kinetics of crystallization was analyzed by integration of the intensity of the Bragg reflections. 7Li NMR relaxometry and pulsed field-gradient (PFG) NMR were used to investigate the short-range and long-range Li+ dynamics in these amorphous and crystalline materials. From the diffusion coefficients obtained by PFG NMR, similar Li+ conductivities for the glassy and heat-treated samples were calculated. For the glassy sample and the glass–ceramic β-Li3PS4 (calcination at 523 K for 1 h), a Li+ bulk conductivity σLi of 1.6 × 10–4 S/cm (298 K) was obtained, showing that for this system a well-crystalline material is not essential to achieve fast Li-ion dynamics. Impedance measurements reveal a higher overall conductivity for the amorphous sample, suggesting that the influence of grain boundaries is small in this case.