First-Principles Characterization and Experimental Validation of the Solid-Solid Interface in a Novel Organosulfur Cathode for the Li-S Battery

Organosulfur silanes grafted on an aluminum current collector have been proposed and demonstrated to function as a sulfur source in the cathode for a lithium-sulfur (Li-S) battery. Bis[3-(triethoxysilyl)propyl]-disulfide silane (TESPD) and bis[3-(triethoxysilyl)propyl]tetrasulfide silane (TESPT) are typical examples of organosulfur complexes used for the study. These organosulfur silanes act as an insulator. Formation of polysulfides (Li2Sx), which is a major bottleneck in the case of elemental sulfur, can be eliminated using this novel cathode. In the absence of charge-carrying polysulfide species, the role of insulating TESPD/TESPT in the charge conduction pathway is an open question. Insight into the interface between the Al current collector and grafted TESPD/TESPT at an atomic level is a prerequisite for addressing the charge conduction pathway. The systematic theoretical methodology is developed based on electronic structure calculations and ab initio molecular dynamics simulations to propose the realistic cathode model (hydration environment) for the Li-S battery. A cluster model is developed to predict the reduction potentials of TESPD/TESPT disclosing the reduction reaction with Li, resulting in the intramolecular S-S bond breaking which is validated by experimental cyclic voltammetry measurements. A realistic cathode model between the aluminum current collector and TESPD/TESPT is also proposed to mimic the experimental conditions where the Al surface was exposed to O-2 and H2O. The top few layers of Al are transformed into alpha-Al2O3 and covered with H2O molecules in the vicinity of grafted TESPD/TESPT. The structural models are further validated by comparing simulated S 2p binding energies with experimental X-ray photoelectron spectroscopy studies.