Charge Transport Reaction Mechanisms Effects Of Catalysts And Co2 Contamination In Rechargeable Non Aqueous Sodium Air Batteries

Metal−air batteries have higher theoretical specific energies than existing rechargeable batteries including state-of-the-art Li-ion batteries. Among metal-air batteries, the Na−O2 battery has gained much attention due to its low discharge/charge overpotentials (< 200 mV) at relatively high current densities (0.2 mA/cm2), high electrical energy efficiency (~90%) and low cost. However, like Li−O2, Na−O2 batteries also suffered from several drawbacks, including dendrite formation, poor rechargeability and low capacity caused by the so-called “sudden death” at its cathode during the discharge process due to insulating discharge products. Therefore, this PhD dissertation is dedicated to investigate the effects that limit cell performance of Na−O2/air batteries. Density Functional Theory (DFT) calculations were employed throughout the study. This work first devoted to estimate the charge transport (ionic conductivity) properties of discharge products (NaO2, Na2O2, Na2CO3) and cathode-electrolyte interfaces (CEI) of non-aqueous Na−O2/air batteries (NASAB). The results revealed that all discharge products are electrical insulators with a large bandgap exceeding 4 eV; however, they offer fast ionic conduction with an activation barrier of < 0.40 eV. Ionic conductivity is mediated by negative sodium vacancies (VNa−). Stable and noble CEI (i.e. NaO2@Na2CO3 and Na2O2@Na2CO3) were designed and their ionic conductivity were investigated. Both interfaces revealed good ionic conduction (with the average rate/diffusion coefficient of 2.62 x 107 𝑠−1/2.96 x 10−8 cm2 s-1 and 1.56 x 104 𝑠−1/1.84 x 10−12 cm2 s-1 for NaO2@Na2CO3 and Na2O2@Na2CO3 interfaces, respectively). The second goal was to evaluate the catalytic activity of boron and nitrogen doped and their co-doped graphene materials towards oxygen reduction/evolution reaction for NASAB. DFT results revealed that boron and pyridinic nitrogen doped graphene exhibited enhanced catalytic performance. The findings could also shed light on developing efficient non-precious carbon-based catalyst materials for metal-air batteries. Finally, the effects of trace CO2 contamination on NaO2 and Na2O2 growth/depletion reaction pathways and overpotentials of the NASAB were investigated on step surfaces. It alters the reaction pathways, lower the equilibrium potential and increases the overpotentials, and thus drastically hampering the performance of the NASAB. Avoiding CO2 contamination is thus critical in the development of NASAB. The computational results of this dissertation would give hints to experimentalists in what to consider during designing and analysis of metal-air batteries, in turn, it will pave the way for the development of highly efficient NASAB.

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