Ionic conduction mechanism of a lithium superionic argyrodite in the Li–Al–Si–S–O system†
We report the conduction mechanism in oxygen-substituted lithium conductors composed of the Li6.15Al0.15Si1.35S6−xOx (LASSO) system, which is a novel member of the argyrodite-type family and has superionic conductivities, making it suitable for all-solid-state batteries. The crystal structures, ionic conductivities, and electrochemical properties of these systems were examined using powder X-ray and neutron diffractometry combined with impedance spectroscopy and cyclic voltammetry measurements. The optimal Li6.15Al0.15Si1.35S5.4O0.6 (x = 0.6) material exhibited a high ionic conductivity of 1.24 mS cm−1 at 25 °C with a low activation energy of 36.6 kJ mol−1. Rietveld refinement and maximum-entropy-method analysis using neutron diffraction data revealed unique interstitial Li+ and O2−/S2− site disorder, which led to a flatter energy landscape for migrating Li+ ions and, thus, a low percolation threshold for three dimensional (3D) Li-ion diffusion. Oxygen substitution also stabilized the structure, and a wide electrochemical window from −0.1 V to 5 V vs. Li/Li+ was achieved. The significant improvements in the ionic conductivity and stability owing to structural changes after cation and anion substitutions reveal an important strategy toward the development of argyrodite-type superionic conductors.