EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Solid-state lithium-sulfur (Li-S) batteries are promising candidates for next-generation energy storage devices with a high theoretical capacity of sulfur. However, there are still challenges to overcome for being competitive with ubiquitous Li-ion batteries. One of the significant challenges is to maintain sufficient charge carrier transport in the composite cathode. As active material sulfur is ionically and electronically insulating, a composite cathode with an optimized volume fractions of conductive additive (typically carbon) and solid electrolyte (highly conductive Li-thiophosphates) is required. Toward high energy density, it is necessary to reduce the total fraction of non-active materials; however, a too little fraction of conductive materials leads to a high overpotential and, with it, poor cycling performance. Besides, Li-thiophosphates in contact with conductive additives can decompose during cycling with its limited electrochemical stability window. The electrolyte degradation may deteriorate ion transport in the composite and it can further degrade the battery performance. Overall, a systematic investigation of carrier transport in the composite is necessary. In this study, charge carrier transport in a cathode composite for solid-state Li-S batteries was investigated as a function of carbon and electrolyte contents, cathode thickness, temperature, and applied potential. The transport properties were measured using effective ionic/electronic conductivity with DC polarization and impedance spectroscopy, indicating that sluggish ion transport in the composites is the rate determining process. Furthermore, compared with stability and cycling tests, the stability window of effective ionic conductivity was found to be larger than the electrochemical stability window of the employed Li-thiophosphate. These results highlight the role of conductive interphases and the impact of their degradation on cycling. In the end, we demonstrated an enhanced cycling performance and achieved a capacity retention of 81.8% in the 100th cycle with 3.68 mAh/cm2