Structural Evolution and Li Dynamics in Nanophase Li₃PS₄ by Solid-State and Pulsed-Field Gradient NMR

Abstract

The ceramic lithium ion conductor β-Li3PS4 has a disordered and nanoporous structure that leads to an enhancement in ionic conductivity by some 3 orders of magnitude compared to the crystalline γ phase. The β phase is prepared by thermal treatment of an inorganic–organic complex based on Li3PS4 and THF. Multinuclear (1H, 6,7Li, 31P) solid-state NMR spectroscopy is used to characterize the structural phase evolution of the starting material at various steps in the thermal treatment. The β phase formed after high temperature treatment is recognized as spectroscopically distinct from the bulk γ-Li3PS4 compound. Also formed is an amorphous lithium thiophosphate phase that is metastable as verified by annealing over an extended period. Lithium ion self-diffusion coefficients are measurable by standard pulsed-field gradient NMR methods at 100 °C and with values consistent with the high ionic conductivity previously reported for this material.