Abstract
The main discharge product at the cathode of non-aqueous Li–air batteries is insulating Li2O2 and its poor electronic conduction is a main limiting factor in the battery performance. Here, we apply density functional theory calculations (DFT) to investigate the potential of circumventing this passivation by controlling the morphological growth directions of Li2O2 using directed poisoning of specific nucleation sites and steps. We show SO2 to bind preferentially on steps and kinks on the (1−100) facet and to effectively lower the discharge potential by 0.4 V, yielding a more facile discharge on the (0001) surface facet. Addition of a few percent SO2 in the O2 stream may be used to control and limit growth of Li2O2 in specific directions and increase the electronic conduction through formation of interfaces between Li2O2 and Li2(SO2)-type inclusions, which may ultimately lead to an increased accessible battery capacity at the expense of a limited increase in the overpotentials.
Highlights
View Article OnlineCommunication the bulk as hole,[15] electron[16] or surface polarons.[11] Luntz et al showed that polaronic transport can signi cantly increase the discharge capacity at low current densities and high temperature.[17] Work by Garcia-Lastra et al.[8] has demonstrated preferential conduction in the directions perpendicular to the [0001] direction, e.g. the [1À100] and [11À20] directions
The main discharge product at the cathode of non-aqueous Li–air batteries is insulating Li2O2 and its poor electronic conduction is a main limiting factor in the battery performance
Addition of a few percent SO2 in the O2 stream may be used to control and limit growth of Li2O2 in specific directions and increase the electronic conduction through formation of interfaces between Li2O2 and Li2(SO2)-type inclusions, which may lead to an increased accessible battery capacity at the expense of a limited increase in the overpotentials
Summary
Communication the bulk as hole,[15] electron[16] or surface polarons.[11] Luntz et al showed that polaronic transport can signi cantly increase the discharge capacity at low current densities and high temperature.[17] Work by Garcia-Lastra et al.[8] has demonstrated preferential conduction in the directions perpendicular to the [0001] direction, e.g. the [1À100] and [11À20] directions This may explain the formation of toroidal Li2O2 particles consisting of stacked Li2O2 platelets with a highly uniform size and shape as observed by Mitchell et al using HRTEM.[18] These platelets reach a thickness of about 5 nm in the [0001] direction and a up to 200 nm in radius in the [1À100] directions.[18] A detailed control of the directions of growth and the Li2O2 morphology is expected to affect and even postpone the onset of sudden death resulting from lack of electronic conduction. To study the reaction of S and SO2 on (1À100) (Fig. 1) and (0001) (Fig. 2) surfaces and the effect their presence has on the Li2O2 growth, we rst calculate the reaction mechanisms for two formula units of Li2O2 at a step on a (1À100) surface, following the approach previously used by Hummelshøj et al.[5,14] This leaves the surface unchanged and prevents energy differences from changing the concentration of surface defects between the initial to the nal state from in uencing the free energy of the overall reaction paths
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