Electrolyte Reactivity on the MgVO 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)) 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 (MgVO) 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 MgVO 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 CF 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 CH or OCH species. The strong stabilization and electron transfer between ionic electrolyte species and MgVO 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 MgVO thin film to form a well-defined electrolyte-MgVO 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 Mg desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgVO 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.