Combined Influence of Meso- and Macroporosity of Soft-Hard Templated Carbon Electrodes on the Performance of Li-O2 Cells with Different Configurations.

Li-O2 batteries can offer large discharge capacities, but this depends on the morphology of the discharged Li2O2, which in turn is strongly affected by the nanostructured carbon used as support in the air cathode. However, the relation with the textural parameters is complex. To investigate the combined effect of channels of different sizes, meso-macroporous carbons with similar mesopore volume but different pore size distribution were prepared from the polymerization of resorcinol-formaldehyde (RF) in the presence of surfactants and micro-CaCO3 particles. The carbon materials were used as active materials of air cathodes flooded by ionic liquid-based electrolytes in Li-O2 cells with two different configurations, one with a static electrolyte and the other with a stirred electrolyte, which favor a film-like and large particle deposition, respectively. The presence of large pores enhances the discharge capacity with both mechanisms. Conversely, with respect to the reversible capacity, the trend depends on the cell configuration, with macroporosity favoring better performance with static, but poorer with stirred electrolytes. However, all mesoporous carbons demonstrated larger reversible capacity than a purely macroporous electrode made of carbon black. These results indicate that in addition to pore volume, a proper arrangement of large and small pores is important for discharge capacity, while an extended interface can enhance reversibility in Li-O2 battery cathodes.

Mass-transport Control on the Discharge Mechanism in Li-O2 Batteries Using Carbon Cathodes with Varied Porosity.

By comparing carbon electrodes with varying porosity in Li-O2 cells, we show that the effect of electrolyte stirring at a given current density can result in a change from 2D to 3D growth of discharged deposits. The change of morphology is evident using electron microscopy and by analyzing electrode pore size distribution with respect to discharge capacity. As a consequence, carbon electrodes with different textural properties exhibit different capacity enhancements in stirred-electrolyte cells. We demonstrate that mass transport can directly control the discharge mechanism, similar to the electrolyte composition and current density, which have already been recognized as determining factors.