Synthesis, air stability and electrochemical investigation of lithium superionic bromine substituted argyrodite (Li6-xPS5-xCl1.0Brx) for all-solid-state lithium batteries

The development of high ionic conducting solid electrolyte is one of the most active research topics in the field of lithium solid-state batteries. In the present study, we enhance the ionic conductivity of the lithium argyrodites using excess halogen substitution at the sulfur site. The halogen-rich Li6-xPS5-xCl1.0Brx (0 < x < 0.5) composition is synthesized through a high-energy ball milling process followed by heat treatment. The ionic conductivity measurement reveals that Li5.6PS4.6Cl1.0Br0.4 solid electrolyte exhibits a higher ionic conductivity value of 9.16 mS cm-1 than Li6PS5Cl (4.96 mS cm-1) at room temperature. The optimized electrolyte is utilized in a solid battery system thus shows a higher capacity of 188 mAh g-1 at 0.1C rate than Li6PS5Cl (181 mAh g-1). Interestingly, Li5.6PS4.6Cl1.0Br0.4 solid electrolyte maintains the higher capacity retention (80.6%) than the Li6PS5Cl (75.3%) after 50 cycles. Further, we demonstrate the influence of excess halogen substitution through electrochemical kinetics using electrochemical impedance spectroscopy and galvanostatic intermittent titration technique. Finally, the air stability test indicates that Li5.6PS4.6Cl1.0Br0.4 electrolyte delivers a high ionic conductivity of 5.26 mS cm- 1 after 30 min dry air exposure.