This study reconstructs pore-scale structures of battery electrodes from scanning electron microscopy images, quantitatively studies the distribution of the electrolyte at various saturations, and simulates the discharge performance of Li-O batteries. This research sheds lights on the critical role of liquid-gas two-phase mass transfer within the porous electrode on the electrochemical performance of batteries. It is found that fully saturated electrodes (100% saturation) have high oxygen-transfer resistance, which will impede the battery performance at typical electrode thickness (∼200 μm). On the contrary, overdried battery (with <50% saturations) electrodes have poor electrochemical performance because dry pores are inactive for electrochemical reactions. In addition, the low electrolyte saturation level leads to low ionic conductivity and high mass transfer resistance of the lithium ion. Carefully designed electrodes with the mixture of lyophilic and lyophobic pores could achieve similar discharge capacity (>7 A h/g) at high current (20 A/m) with lyophilic electrodes that are fully saturated by the electrolyte at low current (1 A/m). The findings from this study enable further research to significantly increase (by orders of magnitude) the operating current and power of the Li-O battery and accelerate its deployment to transport and stationary applications.

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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).

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The wettability of customized Li–O 2 battery electrodes is altered by mixing acetylene black carbon particles with various binders. The wettability of the electrode can be characterized by the static contact angles between the electrode surface and nonaqueous electrolyte, which is 1 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) dissolved in tetraethylene glycol dimethyl ether, and the double-layer capacitance measured by the cyclic voltammetry. Results show that electrodes containing poly(vinylidene difluoride) (PVDF) binder are lyophilic and increasing the fraction of poly(tetrafluoroethylene) (PTFE) increases the lyophobicity of electrodes. Li–O 2 batteries are discharged at 0.1 mA/cm 2 with the cut-off voltage of 2.0 V. The discharge capacity of the electrode with 15% PVDF (36.5°) carbon coatings is 1665.8 mAh/g, whereas the customized electrode with 15% PTFE (128.4°) carbon coatings obtains the discharge capacity of 4160.8 mAh/g. However, the discharge capacity decreases to 3109.5 and 2822.9 mAh/g as the PTFE content further increases to 25% (135.5°) and 35% (138.5°), respectively. The electrode composed of two lyophobic carbon coatings on top and bottom and one lyophilic carbon coating in the middle has the static contact angle of 118.8° and acquires the highest specific discharge capacity of 5149.5 mAh/g.

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,...

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