This study has experimentally investigated effects of the salt concentration in electrolyte on the electrochemical performance of Li-O-2 battery at various current densities. Electrolyte solutions, made from bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME), with different concentrations between 0.005 M and 1 M were tested in the experiment. The viscosity and ionic conductivity of these electrolytes were measured. The first discharge-charge cycle tests were performed on Li-O-2 batteries at current densities from 0.1 to 0.5 mA/cm(2). Both the discharge and charge capacities as well as the columbic efficiency decreased with increasing current density. Results also showed that specific discharge and charge capacities of batteries at very low salt concentration (<= 0.25 M) were extremely low due to the insufficient oxygen and lithium ion and slow diffusion of lithium ion in electrolytes. The balance between the ionic conductivity and mass transfer determines that the optimized salt concentration, when the battery reached the highest discharge/charge capacities, is dependent on the current density. At lower current density (<= 0.2 mA/cm(2)), the highest capacity was obtained with the 0.75 M electrolyte, while at higher current density (0.3-0.5 mA/cm(2)), the highest capacity was obtained with 1 M electrolyte. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.