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 P-31 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 beta-Li3PS4 was observed at temperatures up to 548 K. The kinetics of crystallization was analyzed by integration of the intensity of the Bragg reflections. Li-7 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 beta-Li3PS4 (calcination at 523 K for 1 h), a Li+ bulk conductivity sigma(Li) of 1.6 x 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.