Toward Understanding of the Li-Ion Migration Pathways in the Lithium Aluminum Sulfides Li 3 AlS 3 and Li 4.3 AlS 3.3 Cl 0.7 via 6,7 Li Solid-State Nuclear Magnetic Resonance Spectroscopy.

Li-containing materials providing fast ion transport pathways are fundamental in Li solid electrolytes and the future of all-solid-state batteries. Understanding these pathways, which usually benefit from structural disorder and cation/anion substitution, is paramount for further developments in next-generation Li solid electrolytes. Here, we exploit a range of variable temperature 6 Li and 7 Li nuclear magnetic resonance approaches to determine Li-ion mobility pathways, quantify Li-ion jump rates, and subsequently identify the limiting factors for Li-ion diffusion in Li 3 AlS 3 and chlorine-doped analogue Li 4.3 AlS 3.3 Cl 0.7 . Static 7 Li NMR line narrowing spectra of Li 3 AlS 3 show the existence of both mobile and immobile Li ions, with the latter limiting long-range translational ion diffusion, while in Li 4.3 AlS 3.3 Cl 0.7 , a single type of fast-moving ion is present and responsible for the higher conductivity of this phase. 6 Li- 6 Li exchange spectroscopy spectra of Li 3 AlS 3 reveal that the slower moving ions hop between non-equivalent Li positions in different structural layers. The absence of the immobile ions in Li 4.3 AlS 3.3 Cl 0.7 , as revealed from 7 Li line narrowing experiments, suggests an increased rate of ion exchange between the layers in this phase compared with Li 3 AlS 3 . Detailed analysis of spin-lattice relaxation data allows extraction of Li-ion jump rates that are significantly increased for the doped material and identify Li mobility pathways in both materials to be three-dimensional. The identification of factors limiting long-range translational Li diffusion and understanding the effects of structural modification (such as anion substitution) on Li-ion mobility provide a framework for the further development of more highly conductive Li solid electrolytes.