High-entropy argyrodite glass–ceramic electrolytes for all-solid-state batteries

Abstract

Lithium argyrodite superionic conductors with the general formula Li$_6$PS$_5$X (X = Cl, Br, I) have been intensively investigated in recent years and successfully adopted in the field of solid-state batteries (SSBs). The transport properties of argyrodite solid electrolytes (SEs) usually strongly depend on the degree of occupational disorder. Increasing disorder through complex doping or substitution has been shown to directly affect ionic conductivity. Herein, we explore a high-entropy lithium argyrodite of nominal composition Li$_{6.6}$[P$_{0.2}$Si$_{0.2}$Sn$_{0.2}$Ge$_{0.2}$Sb$_{0.2}$]S$_5$I. This material can be readily prepared by mechanochemistry. Using complementary diffraction techniques, nuclear magnetic resonance spectroscopy, and charge-transport measurements, we show that upon tailoring crystallinity and defect concentration by post-annealing at temperatures up to 220 °C, a high room-temperature ionic conductivity of about 0.9 mS cm$^{−1}$ (∼4.4 mS cm$^{−1}$ bulk conductivity) can be achieved. Both the as-prepared and annealed (at 220 °C) samples were tested in pellet-stack SSB cells. The mechanochemically prepared glass–ceramic SE was found to exhibit superior performance, even outperforming commercially available Li$_6$PS$_5$Cl. Collectively, the results highlight the importance of considering structural aspects across different length scales when optimizing the properties of lithium argyrodites for SSB applications.