(668b) Modeling Hierarchical Li-Air Cathode Designs with Improved Discharge Capacity

The emergence of Li-air battery, particularly non-aqueous Li-air battery have drawn much attention as a promising power source for next-generation electric vehicles. However, prior to its viable commercialization, there are some technical obstacles that need to be resolved (1). Among them, lower discharge capacity is one of the main challenge which is induced by the generation of insoluble lithium peroxide (Li₂O₂), during discharging, inside the cathode pores. Therefore, optimization and designing novel cathodes have become critically acclaimed topic among the scientific community. Extensive experimental and modeling investigations (2-6) have been carried out to develop novel cathode structures. All these studies ended up with the development of functionalized carbon (CNT, CNF, and graphene etc.) cathodes, hierarchical cathode structures functionalized with electrocatalysts (2, 3), and cathode structures with varying wettability (4, 7, 8). These studies focused on enhancing the porosity and pore surface characteristics, resulting in significant improvement in specific discharge capacity. Recently, it is reported (9) that the stochastic nature of pores interconnectivity (called tortuosity) and composite cathodes morphologies may inevitably enhance capacity due to wide range of pore size distribution (micro-, meso-, and macro-pores) and large accessible surface area. Thus, it is deduced that the concept of distributed tortuosity and composite materials also need to be considered in the designing of novel hierarchical cathode structures.

In this presentation, we will present and discuss results concerning a detailed model of Li-Air battery that we have developed and validated in COMSOL. The model helps in examining the potential of hierarchical cathode in guiding and enhancing the efficiency of transport phenomena and discharge product formation inside micro-, meso-, and macro-pores. Discharge curves results of hierarchical cathodes with distributed tortuosity and porosity (higher tortuosity with lower porosity on separator side, and lower tortuosity with higher porosity on cathode airside) considering a discharge current of 0.1 milliampere per square centimeter (mA/cm²) are shown in Figure 1. It is found that this hierarchal cathode have significantly enhanced the discharge capacity by facilitating the effective transport of oxygen and distributing Li ions along the pores which in turn avoid blocking the pore space. This implies that hierarchical cathode materials with distributed tortuosity and porosity improved the pore alignment, effective transport of oxygen, active reactive area, and space to accommodate the solid discharge product. Despite containing same average porosity (Figure 1b), different porosity distributions resulted in different specific discharge capacities because of different utilization rates of their corresponding pore systems. It is evident that the discharge capacity attained in exponential case (red line) is larger than that in uniform initial porosity (dark gray line). This is because the exponential distribution in porosity effectively facilitates the pore distribution, pore volume, and accessible surface area resulting in facile transport and maximum accommodation of Li₂O₂ compared to the rest of the cases.

We acknowledge financial support for this work from Khalifa University of Science and Technology (projects RC2-2019-007 and CIRA2018-103).

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