Lithium–sulfur cells with a sulfide solid electrolyte/polysulfide cathode interface

Abstract

Lithium–sulfur batteries use a solid electrolyte as an alternative to the conventional liquid electrolyte to form solid-state lithium–sulfur cells with a high energy density. Sulfur cathodes are the common active material used in conjunction with solid electrolytes. However, their insulating nature and the solid–solid interface with the solid electrolyte result in poor cyclability and low active material loading. In this study, we adopt a polysulfide cathode and a sulfide solid electrolyte to generate a novel lithium-sulfur cell configuration. The liquid–solid interface provided by the polysulfide catholyte allows a close connection between the electrode and electrolyte, enhancing charge transfer and lithium-ion diffusion. The improved interface means that the polysulfide cathode exhibits high reactivity, allowing us to achieve a high sulfur loading (5 mg cm⁻²), a high areal capacity (5.1 mA h cm⁻²), and a long cycle life (approaching 100 cycles). X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy confirm the formation of lithium phosphide and lithium phosphate on the cathode side of the electrolyte. These compounds form an ionically conductive protection layer that optimizes the contact between the polysulfide cathode and the solid electrolyte and stabilizes the active electrodes, thereby enhancing the kinetics and stability of the electrochemical reaction.