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In the last decade, great effort has been paid to the development of next generation batteries. Metal-O2 /Air batteries (Li-, Na-, Mg-, Al-, Fe- and Zn-O2 batteries) in both aqueous and nonaqueous (aprotic) electrolytes have gained much attention. Metal-air batteries have high theoretical specific gravimetric energy. In the case of Li-O2, it is comparable to that of gasoline. Thus, Li-O2 batteries could be attractive for electric vehicle manufacturers since the energy storage capacity accessible by commercially available Li-ion technology is too low to solve increasing capacity demands. However, current Li-O2 batteries suffer from several drawbacks, e.g. 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. This thesis is devoted to understand the charge transport in the main reaction products of emerging nonaqueous Li- and Na-O2 batteries at the atomistic level using the Density Functional Theory (DFT) method to address the latter problem. The role of cathode-electrolyte interface on charge transport and the implication of impurities from the air, particularly the effect of CO2 poisoning, in the performance of the battery are addressed. The present work involves computational investigations of different charge transport mechanisms, i.e. ionic, coherent electron, and polaronic transport. In order to validate the outcome from DFT calculations, results are compared with relevant experiments and show a notable agreement. The results of charge transport calculations in bulk Li2O2 (main discharge product in Li-O2 batteries) revealed that though it is a wide bandgap insulator (4.96 eV) it could offer fast ionic conduction with an activation barrier of 0.40 eV. Similarly, an accessible energy barrier for sodium ion diffusion is obtained in Na2O2 and in NaO2 (main discharge products in Na-O2 batteries). The transport mechanisms at the cathode-electrolyte interfaces, i.e. Li2O2@Li2CO3 interface, are also examined. Lithium vacancies accumulate at the peroxide side of this interface, reducing the coherent electron transport by two to three orders of magnitude compared to bulk pristine Li2O2. In contrast, the Li2O2@Li2CO3 interface shows an improved ionic conduction. For polaronic transport significant differences are also found in these two scenarios. In bulk Li2O2 the polaronic transport at room temperature is restricted to hole polarons, whereas electron polarons display very high hopping barriers (> 1.0 eV). By contrast, it is possible to have good mobilities for electron polarons at the Li2O2@Li2CO3 interface. Finally, our studies on the reaction mechanism of Li2O2 revealed that the CO2 poisoning, even at low concentrations of CO2 effectively blocks the step nucleation site and remarkably increases overpotentials and decreases the capacity of the battery.
|Publisher||Department of Energy Conversion and Storage, Technical University of Denmark|
|Number of pages||143|
|Publication status||Published - 2015|
01/08/2012 → 02/12/2015