Electrolyte Reactivity on the MgV 2 O 4 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 (MgV 2 O 4 ) 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 MgV 2 O 4 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 3 - 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 3 + or OCH 3 - species. The strong stabilization and electron transfer between ionic electrolyte species and MgV 2 O 4 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 MgV 2 O 4 thin film to form a well-defined electrolyte-MgV 2 O 4 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 x , carbonates) and the higher amount of MgF x with [Mg(TFSI):G2] + formed in the interfacial region, which corroborates the theoretical observation. Overall, these results indicate that Mg 2+ desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgV 2 O 4 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.