Superionic lithium conductor with a cubic argyrodite-type structure in the Li–Al–Si–S system

Abstract

All-solid-state lithium batteries have been proposed as solutions to safety issues associated with conventional lithium batteries; consequently, their development is important. To that end, we conducted a material search of solid electrolytes in the Li4+xAlxSi1−xS4 system. High-temperature (HT) and low-temperature (LT) modifications of novel argyrodite-structured phases were obtained at around x = 0.1 by melt quenching. The HT-argyrodite phase showed the highest ionic conductivity (2.5 × 10−4 S cm−1) at 28 °C, with an activation energy of 40 kJ mol−1; this conductivity value is three orders of magnitude higher than those of Li4+xAlxSi1−xS4 solid solutions. The relationship between ionic conductivity and phase-type was elucidated. An all-solid-state lithium battery using an HT-argyrodite-phase electrolyte exhibited a discharge capacity of over 120 mAh g−1 and an excellent discharge efficiency of about 100% after the second cycle, demonstrating that this material is suitable for use in practical all-solid-state batteries.