PNNL

Abstract

Predictive understanding of the solvation-dependent reactivity and molecular interaction of electrolyte ions and solvent molecules on reactive electrodes has been a major challenge but is essential for addressing instabilities and surface passivation that occur at electrode-electrolyte interface (EEI) of multivalent Mg 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]+, [Mg(TFSI):2G2]+) as well as neutral molecules (G2) on magnesium vanadate cathode (MgV2O4) surface has been studied using a combination of first-principles calculations and multimodal analysis of well-defined cathode electrolyte interphase (CEI) layers. Our calculations show that non-solvated [Mg(TFSI)]+ is the strongest adsorbing species on the MgV2O4 surface compared to all other ions while fully solvated [Mg(TFSI):2G2]+¬ are least favorable to decomposition. The cleavage of C-S bonds in TFSI- to form CF3- is predicted to be 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 to 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 MgV2O4 thin film to form well-defined electrolyte-MgV2O¬4 interface. Analysis of soft-landed interphase 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., MgFx, carbonates) formed in the interfacial region and the higher amount of MgFx with [Mg(TFSI):G2]+, which corroborates the theoretical observation. Overall, we established the mechanistic pathway for the electrolyte-induced formation of passivating fluorides on MgV2O4 cathode facilitated by the surface adsorption and charge transfer, which provided essential knowledge for rational design of stable electrolytes for multivalent cathodes.