This study experimentally investigates and numerically simulates the influence of the cathode electrode open ratio (ratio of oxygen-opening area to the total electrode surface area) on the performance of Li-O batteries at various discharge current densities. At the current density of 0.1 mA/cm, the maximum discharge capacity is achieved at 25% open ratio among the tested open ratios (0-100%). As the open ratio increases from 25% to 100%, the specific discharge capacity decreases from 995 to 397 mA h/g. A similar trend is observed at 0.3 mA/cm, while the maximum discharge capacity is obtained at 3% open ratio among the tested open ratios. The model that assumes the electrode is always fully saturated by the electrolyte does not obtain similar trends with experimental results, while the model that considers electrolyte loss by evaporation and the volume change of the solid obtains the same trend with experimental observations. The open ratio governs not only availability of oxygen but also the evaporation of the electrolyte and the contact resistance. The faster evaporation of the electrolyte at a higher open ratio can be the main reason for the decrease of the discharge capacity, especially when the open ratio is relatively high (above 25%). Meanwhile, the contact resistance of the battery, measured by the electrochemical impedance spectroscopy (EIS), increases from 3.97 to 7.02 Ω when the open ratio increased from 3% to 95%. The increase of the Ohmic overpotential, however, is negligible (on the order of millivolts) because of the low discharge and charge current rates (on the order of 0.1 mA).

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. Li-O 2 batteries have received significant interest as one of the most promising technology for energy storage in the past few years due to its high theoretical energy density (1700 Wh/kg) compared with those of Li-ion batteries.1-3 Abraham and Jiang 4 first reported a Li-O 2 battery using organic electrolytes since the Li-O 2 aqueous electrolyte batteries suffered from metal corrosion by water. Generally, a rechargeable organic electrolyte Li-O 2 battery is composed of a lithium metal anode, a separator saturated with the organic electrolyte, and a porous cathode electrode (typically made from carbon or catalysts). During discharge, the lithium metal is oxidized to lithium ions at the anode, shown as Eq. 1. Meanwhile, oxygen from the surrounding dissolves in the liquid electrolyte, reacts with lithium ion, and generates solid Li 2 O 2 in the cathode electrode, which are shown as Eq. 2. During charge, the reversed cathodic reaction decomposes lithium peroxide and releases oxygen and lithium ion. The reversed anodic reaction deposits lithium metal at the anode electrode. The overall reaction is shown in Eq. 3 and the theoretical voltage, E 0 , of the reaction is 3.1 V. Cathode : 2LiSame electrochemical reactions take place in Li-air batteries, 6,7 in which O 2 is breathed from the ambient air. Since CO 2 and H 2 O in air would react with active components in batteries and deteriorate the performance, most laboratory experiments were conducted under pure O 2 environment. This experimental study was also carried out using pure oxygen and the term Li-O 2 battery is used throughout this paper. [8][9][10][11] Researches that focus on electrolyte solvents, lithium salts,...

Developing batteries with high specific capacity and power density is essential in many applications such as electric vehicles and portable electronic devices. The Li-oxygen battery has a very high theoretical energy density of 11 kWh kg-1 and is considered as a promising battery technology. The concentration of the lithium ion in battery electrolyte is typically 1 M in both Li-ion and Li-oxygen batteries. Considering the high cost of the lithium salt and low current rates of Li-oxygen battery, this study investigated effects of salt concentration (LiPF6 in Tetraethylene glycol dimethyl ether) on battery performance through experiments and model simulations. Results showed that when the salt concentration decreased from 1 M to 0.05 M, the specific discharge capacity decreased from 557.7 mAh/g to 390.9 mAh/g, and the specific charge capacity also decreased from 512.7 mAh/g to 339.5 mAh/g at the current rate of 0.1 mA/cm2. Both the discharge and charge capacity decreased significantly to less than 31.1 mAh/g when the lithium salt concentration was lower than 0.005M. The electrolyte conductivity that can affect the ion transport in the electrolyte was also measured in the experiment. The measured conductivity decreased from 3010.0 μS/cm to 16.6 μS/cm when the concentration of electrolyte decreased from 1 M to 0.005 M, while the molar conductivity was almost constant (3.518 +/- 0.048 S.cm2/mol) in this concentration range. A one-dimensional model was also developed to illustrate the importance of mass transfer in the electrode and simulate the electrochemical performance of Li-oxygen batteries.