Electrolytes are key components of Li/O-2 batteries. The ionic liquid-like structure and good electrochemical and thermal stability of solvent-in-salt electrolytes make them of great interest for lithium batteries. Solutions of lithium bis(trifluoromethanesulfonyl) imide in tetraethylene glycol dimethyl-ether with molar ratios ranging from 1:9 to 1:0.9 are here investigated. A voltammetric study of oxygen redox reaction in presence of different concentrations of salt, from salt-in-solvent to solvent-insalt solutions, is reported here for the first time along with a novel luminescence method for the evaluation of O-2 solubility. The results indicate that superconcentrated solutions favor the solution formation mechanism of Li2O2 during discharge which in turn is beneficial for battery cycling stability. Despite the higher viscosity of solvent-in-salt solutions than conventional electrolytes, O-2 solubility is improved at the highest salt concentrations. These findings contribute to understand electrochemical processes in solvent-in-salt solutions for Li/O-2 and next generation metal-based batteries. (C) 2017 Elsevier Ltd. All rights reserved.