Non-Aqueous Li-Air Flow Battery: Improving Capacity and Current Density through Cathode Optimization

Abstract

Non-aqueous Lithium-air (Li-air) batteries are attractive energy storage systems due to their exceptionally high theoretical energy density. Despite its high energy density, issues such as cycle fading and low current density still need to be overcome. A major cause of these issues is the deposition of discharge products on the surface as well as inside the pores of the cathode. These deposits clog the pores and prevent continuous steady discharge. Recently we developed a Li-air battery configuration that has a flowing Li-ion free ionic liquid as the electrolyte, enabling the continuous removal of discharge products (Figure 1). This flow battery consists of two units, a flow battery for electrochemical reaction and a storage site for discharge products. In detail, a gas diffusion electrode (GDE) allows continuous supply of O2while fresh catholyte is continuously provided to transport discharge products away to the storage. Also, the Li metal anode is separated from the catholyte by a solid state Li-ion conducting separator. We successfully demonstrated separation of the electrochemical reaction on the cathode and storage of the discharge products by employing a flowing catholyte. This approach led to the flowing Li-air battery exhibiting a higher discharge capacity than a static Li-air battery (i.e. with a non-flowing electrolyte configuration). However, the discharge current density achieved with this Li-air flow battery was still low. To improve the discharge current density of the Li-air flow battery, we used COMSOL modelling to determine parameters that are key in optimizing battery performance modeling in COMSOL. We simulated a modified version of Li-air battery model, based on a model presented in prior work.1 This model identified optimal parameters for the cathode, including the porosity of electrode and the depth of immersion of the cathode with electrolyte. The maximum current density that can be achieved increases as the porosity of the cathode increases because a higher porosity improves O2 transport. However, a different trend was observed for the depth by which the electrolyte had penetrated the cathode. The maximum current density is observed at 30 and 50 µm for an initial concentration of O2 of 5 and 15 mol m3, respectively, suggesting that an optimum electrolyte immersion depth is key to achieve the highest current density. In parallel with these modeling efforts, the structure and surface of GDEs were modified and evaluated to increase discharge current density for Li-air flow battery. Specifically, we fabricated and evaluated hot-pressed GDEs, super-hydrophobic GDEs coated with PTFE, and GDEs on which additional material had been deposited. Especially the hot-pressed GDEs exhibited improved discharge current density, which we attribute to the hot-pressing decreasing the thickness of GDEs, thus enabling better O2transport. In summary, we established the concept of a Li-air battery with a flowing Li-ion free non-aqueous electrolyte. Also, we used modeling and experiments to analyze how different parameters, specifically the properties of the cathode, benefit the performance of the non-aqueous Li-air flow battery. We were able to optimize the gas diffusion electrode-based cathode for this Li-air flow battery, which lead to higher discharge current densities.