Electrolyte Reactivity on the MgV₂O₄ Cathode Surface

Predictive understanding of the molecular interaction of electrolyte ions and solvent molecules and their chemical reactivity on electrodes has been a major challenge but is essential for addressing instabilities and surface passivation that occur at the electrode-electrolyte interface of multivalent magnesium batteries. In this work, the isolated intrinsic reactivities of prominent chemical species present in magnesium bis(trifluoromethanesulfonimide) (Mg(TFSI)₂) in diglyme (G2) electrolytes, including ionic (TFSI^-, [Mg(TFSI)]⁺, [Mg(TFSI):G2]⁺, and [Mg(TFSI):2G2]⁺) as well as neutral molecules (G2) on a well-defined magnesium vanadate cathode (MgV₂O₄) surface, have been studied using a combination of first-principles calculations and multimodal spectroscopy analysis. Our calculations show that nonsolvated [Mg(TFSI)]⁺ is the strongest adsorbing species on the MgV₂O₄ surface compared with all other ions while partially solvated [Mg(TFSI):G2]⁺ is the most reactive species. The cleavage of C-S bonds in TFSI^- to form CF₃^- is predicted to be the most desired pathway for all ionic species, which is followed by the cleavage of C-O bonds of G2 to yield CH₃⁺ or OCH₃^- species. The strong stabilization and electron transfer between ionic electrolyte species and MgV₂O₄ is found to significantly favor these decomposition reactions on the surface compared with intrinsic gas-phase dissociation. Experimentally, we used state-of-the-art ion soft landing to selectively deposit mass-selected TFSI^-, [Mg(TFSI):G2]⁺, and [Mg(TFSI):2G2]⁺ on a MgV₂O₄ thin film to form a well-defined electrolyte-MgV₂O₄ interface. Analysis of the soft-landed interface using X-ray photoelectron, X-ray absorption near-edge structure, electron energy-loss spectroscopies, as well as transmission electron microscopy confirmed the presence of decomposition species (e.g., MgF, carbonates) and the higher amount of MgF with [Mg(TFSI):G2]⁺ formed in the interfacial region, which corroborates the theoretical observation. Overall, these results indicate that Mg²⁺ desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgV₂O₄ cathode surface. This work provides the first evidence of the primary mechanisms leading to electrolyte decomposition at high-voltage oxide surfaces in multivalent batteries and suggests that the design of new, anodically stable electrolytes must target systems that facilitate cation desolvation.