Experimental and Numerical Investigations of Wettability of Positive Electrodes for Li−O2 Batteries

Abstract

The objective of this dissertation is to characterize the positive electrode wettability and its effects on the performance (e.g., discharge capacity) of Li−O2 batteries. The investigations include an experimental study of discharging electrodes with various wettabilities, proposing and examining the intermittent discharge strategy, and the numerical simulation of the distribution of the electrolyte at various saturations and of the discharge performance of Li−O2 batteries at the pore scale. Future work will measure the structure of positive electrodes using advanced imaging technology such as transmission X-ray microscopy. First, I fabricated the electrodes and adjusted their wettability by mixing acetylene black carbon particles with various binders. The wettability was quantitatively characterized by the contact angle and ionic resistance. The customized electrodes were then discharged in Li−O2 batteries at 0.1 mA/cm2 through which the relationship between electrode wettability and discharge capacity was obtained. The discharge capacity of the electrode with 15% PVDF (36.5°) binder was 1665.8 mAh/g while the customized electrode with 15% PTFE (128.4°) binder had a discharge capacity of 4160.8 mAh/g. The effects of lyophobicity on O2 transfer in the porous electrode have been proved. A positive electrode with mixed wettability was designed and tested, which acquired the highest specific discharge capacity of 5149.5 mAh/g. The structure of this electrode included two lyophobic carbon coatings on top and bottom and one lyophilic carbon coating in the middle. Further design may focus on appropriately configuring the wettability to balance the gas paths for O2 diffusion and wetted area for reaction sites. A novel strategy for discharging Li−O2 batteries was then proposed and identified. The battery was periodically discharged and rested, which can enhance O2 availability and increase the discharge capacity. Periodically resting the battery increased the specific discharge capacity by at least 50% at various current densities (0.1 - 1.5 mA/cm2). Afterward, the investigation combined the electrode wettability and the intermittent strategy. Compared with the continuous strategy, the capacity of lyophobic electrodes increased by over 100% when the intermittent strategy was applied. Besides, a multi-step discharge strategy can provide greater capacity when the battery is discharged at decreasing current rates (2.0, 1.5, and 1.0 mA/cm2). The importance of O2 diffusion is emphasized and provide practical strategies are proposed to improve the deep discharge capacity of Li-O2 batteries, especially at high current rates (> 1.0 mA/cm2). Finally, a numerical study was conducted to investigate the electrode with different saturations of the electrolyte. The effects of electrolyte saturation levels and the distribution of electrolyte have been demonstrated by comparing the corresponding discharge performance of Li-O2 batteries. It was found that fully saturated electrodes (100% saturation) have high oxygen transfer resistance, which will result in the lowest discharge capacity of 7.41 Ah/g. On the contrary, over-dried battery (with 1.0 mA/cm2). Finally, a numerical study was conducted to investigate the electrode with different saturations of the electrolyte. The effects of electrolyte saturation levels and the distribution of electrolyte have been demonstrated by comparing the corresponding discharge performance of Li-O2 batteries. It was found that fully saturated electrodes (100% saturation) have high oxygen transfer resistance, which will result in the lowest discharge capacity of 7.41 Ah/g. On the contrary, over-dried battery (with 7 Ah/g) at high current (20 A/m2) similar to hydrophilic electrodes which are fully saturated by the electrolyte at low current (1 A/m2). The modeling study found that designing the electrode with a mixture of lyophilic and lyophobic pores is critical to significantly increasing (by orders of magnitude) the operating current and power of the Li–O2 battery. In the future, plans are to characterize the geometry of the positive electrode using the imaging techniques (e.g., transmission X-ray microscopy) and gas sorption method. Based on the characterization of the porous structure, the relationship between the porous structure and the mass transport phenomena will be clarified.