Abstract
Understanding the reactivity of electrolyte and solvent molecules as well as electroactive species at operating battery interfaces enables the rational design of improved battery technologies. This molecular-level understanding, however, is often challenging to obtain due to the heterogeneity of electrode surfaces and the complexity of chemical transformation occurring under applied electric fields. For example, battery electrode surfaces contain various distributions of metal-oxides, -hydroxides, -carbonates, and -halides. Subsequently, redox and electron transfer properties vary widely over these heterogeneous surfaces. In this work, we provide a comprehensive outlook and delineate the reactivity of selected polysulfide electroactive ions on well-defined reactive lithium surfaces and propose an approach to modulate the surface reactivity by using electron-accepting surface layers. We used ion soft landing to deposit mass-selected lithium polysulfide ions (LiS4- and Li5S12-) on oxidized and fluorinated lithium surfaces. Ion soft-landing is a versatile mass spectrometry-based deposition technique that allows ionic molecules to be selectively deposited onto electrode surfaces without counter-ions and solvent molecules. The reactivity of the deposited polysulfide ions was studied using a combination of experimental and theoretical approaches. Specifically, in-situ X-ray photoelectron and Raman spectroscopy revealed the presence of both reduced (Li2S) and oxidized (LixSOy) polysulfide species on electron-accepting lithium oxide surfaces. Ab-initio molecular dynamics calculations corroborated the formation of species observed in the experimental study. Bader charge analysis of the systems showed multi-electron transfer reactions for sulfur atoms, starting from neutral species and reacting to oxidation states ranging from +3 to -2. Calculations also indicate that a relatively thick (> 1 nm) surface oxide layer is required to promote reactivity, and that thinner layers can inhibit reaction. Our study provides fundamental insights into the complex redox chemistry and decomposition mechanism that occurs at heterogeneous lithium electrodes and will lead to rational design of efficient and stable battery interfaces.