Degradation of LiTfO/TEGME and LiTfO/DME electrolytes in Li-O2 batteries

In this work we discuss the degradation chemistry on carbon-free electrodes of two ether based electrolytes for Li-O2 batteries, i.e. tetraethylene glycol dimethyl ether (TEGDME) and dimethoxy ethane (DME) with lithium trifluoromethane sulfonate (LiTfO) as salt. To this aim we developed an all-metallic positive electrode by electrodeposition of a gold dendritic film on a nickel foam (Au@Ni). These carbon-free electro-catalytic electrodes have been used to investigate the degradation chemistry of the electrolytes in Li-O2 cells by eliminating the parallel parasitic reactions due to the commonly used carbon electro-catalysts. In particular the composition and morphological evolutions of the Au@Ni electrodes after discharge and cycling have been characterized ex situ by Raman Spectroscopy, X-ray Photoemission Spectroscopy and Scanning Electron Microscopy. We also couple this experimental study with thermodynamic predictions about the onset degradation of the DME molecule based on density functional theory calculations. In summary in both DME/LiTfO and TEGDME/LiTfO electrolytes, the degradation involves the oxidation of the ether solvent to a mixture of carbonates and carboxylates/formate/oxalate. DME is apparently more strongly degraded compared to TEGDME whereas the LiTfO anion is highly stable. Calculations suggest the key role played by the singlet oxygen molecule as initiator of the degradation path.