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

The ceramic lithium ion conductor beta-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 gamma phase. The beta phase is prepared by thermal treatment of an inorganic-organic complex based on Li3PS4 and THF. Multinuclear (H-1, Li-6,Li-7, P-31) solid-state NMR spectroscopy is used to characterize the structural phase evolution of the starting material at various steps in the thermal treatment. The beta phase formed after high temperature treatment is recognized as spectroscopically distinct from the bulk gamma-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 degrees C and with values consistent with the high ionic conductivity previously reported for this material.