Hongli Zhu1
Northeastern University1
All-solid-state lithium-sulfur batteries (ASSLSBs) hold promise for high energy density and improved safety compared to conventional lithium-sulfur cells using liquid electrolytes. However, low utilization of active sulfur caused by sluggish reaction kinetics has greatly hindered ASSLSB development. Achieving efficient sulfur electron/ion accessibility in the cathode structure is critical. Porous carbon hosts, widely used in liquid cells, have been proposed to address these challenges. However, conventional porous carbons with buried pores are ineffective for ASSLSBs, as the non-mobile solid electrolytes cannot penetrate the pores to access enclosed sulfur. An ideal porous carbon should maximize surface area for sulfur while restricting pores to only the exterior surface. Despite works applying porous carbons in ASSLSBs, the optimal structure has not been well elucidated. Here, we pioneer discussion on the ideal carbon structure and develop polyacrylonitrile-derived porous carbon fibers (PPCF) with a unique core-shell morphology. A microporous shell on a dense core provides high surface area with accessible pores for electrolytes and sulfur. Consequently, ASSLSBs with PPCF show outstanding performance.<br/>Furthermore, we grow MoS2 nanosheets on carbon fibers. The chemical and electrochemical compatibility of MoS2 with sulfur and sulfide solid electrolytes greatly improves cathode stability and ion/electron transport. Metallic 1T MoS2 enables electron transfer, while the layered structure facilitates Li intercalation. This significantly enhances kinetics and relieves electrolyte decomposition. Our optimized ASSLSB delivers an ultrahigh initial capacity of 1456 mAh/g with high coulombic efficiency and 78% retention after 220 cycles.<br/>Finally, we reveal the Li-S redox reaction undergoes a two-step transformation, producing polysulfide intermediates. Kinetic limitations can cause incomplete conversions, leaving polysulfides like Li2S2. Our mechanistic insights guide design principles for ASSLSBs.