Sulfone-Based Electrolytes for Nonaqueous Li-O2 Batteries

We investigated the use of sulfone-based electrolytes for the Li-O-2 battery. The study compared the behavior of three commercially available sulfones: ethyl vinyl sulfone (EVS), tetramethylene sulfone (TMS), also called sulfolane, and ethyl methyl sulfone (EMS). First, we carried out a preliminary investigation of the oxygen reduction reaction and oxygen evolution reaction (ORR/OER) as a function of solvent type and Li+ concentration. Then, TMS and EMS were tested (LiTFSI salt) in Li-O-2 cells. The cells exhibited initial capacities around 1800 and 2000 mAh.g(carbon)(-1), respectively. The capacity retention on cycling was quite low. We analyzed the reaction products during discharge and charge by means of powder X-ray diffraction, infrared spectroscopy, H-1-nuclear magnetic resonance, and mass spectrometry. Although EVS was at first sight the most attractive sulfone, since it is a liquid at room temperature, it was the least stable in the presence of oxygen; its vinyl group was attacked by reduced O-2 species. On the other hand, both TMS and EMS performed better during the first five cycles; Li2O2 formation and decomposition was the main reaction, although some byproducts formed during cycling. After five cycles, there was still a considerable amount of Li2O2 formed, but decomposition to form Li2CO3 became significant, and it accumulated at the O-2 electrode. This was the likely reason for capacity fading.