Li4PS4I: A Li+ Superionic Conductor Synthesized by a Solvent-Based Soft Chemistry Approach

A novel crystalline lithium superionic conductor, Li4PS4I, has been discovered utilizing a solvent-based synthesis approach. It was found that the starting material Li3PS4.DME reacts with LiI in a 1:1 ratio in DME to give a precursor that results in Li4PS4I after soft heat treatment at around 200 degrees C in vacuum. Its crystal structure was solved ab initio by evaluating both X-ray (Mo-K alpha(1)) and neutron (TOP, GEM, ISIS) powder diffraction data, in a combined refinement (P4/nmm, Z = 2, a = 8.48284(12) angstrom, = 5.93013(11) angstrom, wR(p) = 0.02973, GoF = 1.21499). The final structure model comprises, besides Li+ ions, isolated PS(4)(3-)tetrahedra in a layer-like arrangement perpendicular to the c-axis that are held apart by I- ions. The Li+ ions are distributed over five partially occupied sites residing in 4-, 5-, and 6-fold coordination environments. A topostructural analysis of the voids and channels within the PS4I4- substructure suggested a threedimensional migration pathway system for the Li+ ions in Li4PS4I. The Li+ ion mobility was studied by temperature-dependent impedance spectroscopy as well as Li-7 solid-state nuclear magnetic resonance (NMR) spectroscopy including the measurement of spin-lattice relaxation rates T-1(-1). The total ionic conductivity was determined to be in the range of 6.4 X 10(-5) to 1.2 X 10(-4) S.cm(-1) at room temperature with activation energies (E-A) of 0.37 to 0.43 eV. The NMR analyses revealed a hopping rate of the Li+ ions of tau(-1) = X 10(8) s(-1) corresponding to a bulk conductivity of 1.3 x 10(-3) S.cm(-1) at 500 K and an activation energy E-A = 0.23(1) eV.