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 Li 2 S–P 2 S 5 glasses. Herein, the amorphization process of Li 2 S–P 2 S 5 (75:25) induced by ball milling was analyzed via X-ray diffraction (XRD), Raman spectroscopy, and 31 P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. Several structural building blocks such as [P 4 S 10 ], [P 2 S 6 ] 4–, [P 2 S 7 ] 4–, and [PS 4 ] 3– occur during this amorphization process. In addition, high-temperature XRD was used to study the crystallization process of the mechanically milled Li 2 S–P 2 S 5 glass. Crystallization of phase-pure β-Li 3 PS 4 was observed at temperatures up to 548 K. The kinetics of crystallization was analyzed by integration of the intensity of the Bragg reflections. 7 Li 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-Li 3 PS 4 (calcination at 523 K for 1 h), a Li + bulk conductivity σ Li 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.