Selective Blockage of Li-Ion Diffusion Pathways in Li10SnP2S12: Insights from Nuclear Magnetic Resonance

The fast ionic conductor Li10GeP2S12 (LGPS) exhibits exceptionally high ionic conductivity at room temperature, which is attributed to the two-dimensional (2D) pathways that connect the one-dimensional (1D) Li-ion fast diffusion tunnels. These anisotropic transport pathways for Li ions can be modulated by isovalent substitution of the Ge element by Sn or Si. In this work, we use multiple 7Li and 31P solid-state nuclear magnetic resonance (ss-NMR) methods to study the effects of Ge substitution with Sn on 1D and 2D Li-ion dynamics as well as conductivity. Like its analogue LGPS, Li10SnP2S12 (LSPS) also exhibits two distinguishing Li-ion diffusion paths, i.e., a 1D channel along the c-axis and 2D ab-plane diffusion, characterized by activation energies of 0.17 eV at low temperatures and 0.13 eV at high temperatures. It is interestingly found that replacing Ge with Sn blocks most of the Li(4)–Li(1) diffusion pathways in the ab-plane and only approximately 10% of the Li(4) sites on average remain active for in-plane Li diffusion. This element substitution, however, does not significantly modulate the 1D in-channel Li-ion diffusion. Such a substitution-induced heavy blockage of the in-plane Li(4)–Li(1) ion diffusion pathways is thus responsible for the reduced room-temperature conductivity of LSPS. Our results provide a new insight toward understanding the structure–conductivity relationship and the strategy of tuning the ionic conductivity.