1,2-Dimethoxyethane Degradation Thermodynamics in Li-O2 Redox Environments.

The reaction thermodynamics of the 1,2-dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li-O2 rechargeable batteries, has been studied by first-principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O2- in oxygen-poor or oxygen-rich environments is studied by density functional calculations. Solvation effects are considered by employing a self-consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen-poor environments and methyl oxalate, methyl formate, 1-formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen-rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li-O2 cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O2- /O2 molecules. These findings pave the way for a rational development of new solvent molecules for Li-O2 electrolytes.