Direct Observation of Hybridization Between Co 3d and S 2p Electronic Orbits: Moderating Sulfur Covalency to Pre-Activate Sulfur-Redox in Lithium-Sulfur Batteries.

Abstract

Lithium-sulfur batteries (LSBs) offer high energy density and environmental benefits hampered by the shuttle effect related to sluggish redox reactions of long-chain lithium polysulfides (LiPSs). However, the fashion modification of the d-band center in separators is still ineffective, wherein the mechanism understanding always relies on theoretical calculations. This study visibly probed the evolution of the Co 3d-band center during charge and discharge using advanced inverse photoemission spectroscopy/ultraviolet photoemission spectroscopy (IPES/UPS), which offers reliable evidence and are consistent well with theoretical calculations. This, coupled with in situ Raman and X-ray diffraction (XRD) and electrochemical data, co-evidences a novel pre-activating S redox mechanism in LSBs: LiPSs desert/insert in C-N matrixes within a series of Co@NCNT-based separators. The insight of the S redox pre-activation is discovered that the Co 3d-band center downshifts to hybridized with S 2p orbitals in LiPSs, giving rise to a more pronounced S covalency and thus accelerating the conversion of LiPSs to S₈. Benefiting from these advantages, the optimized LSB possesses a minimal decay rate of 0.0058% after 200 cycles at a high discharge rate of 10 C. This study provides new insights into LSB mechanisms and supports conventional theoretical models of the d-band center's impact on LSB performance.