Electrospun carbon fibers as air cathodes for aprotic Li-O2 battery: towards cathode design for enhanced capacity

ABSTRACT This report investigates how the microstructures and chemical properties of carbon cathodes influence on the discharge capacity of aprotic Li-O2 batteries. For that, electrospun carbon fibers (CFs), Multi-wall carbon nanotubes (MWCNTs), carbon Super P and gas diffusion layer (GDL) were fully discharged at various applied current densities (0.05 - 0.5 mA.cm-2(geom)) and characterized by means of ex-situ techniques such as XRD and FEG−SEM. The major discharge product for every carbon electrode was identified as Li2O2 by both XRD and gas pressure analysis. The corresponding electrochemical results showed two different behaviors depending on the current density. At high current densities, all the carbon electrodes presented quite similar discharge capacities due to the most favored formation of Li2O2 conformal film on the carbon surface. In contrast, at lower current densities, the morphology of the discharge product changed and Li2O2 was preferably deposited as micrometer−sized toroids particles on all the carbon cathodes (except for GDL). In this particular regime, the specific capacity normalized by the active carbon surface area of the electrospun CFs cathode was surprisingly promoted compared to the other carbon materials. Interestingly, it was found that unlike for the other cathode materials, Li2O2 homogenously occupies the void volume of the electrospun carbon structure without any pores clogging, which was attributed to its particular macroporous architecture presumably facilitating a continuous O2 diffusion through the electrode. The enhancement in discharge capacity might also be related to the presence of nitrogen active sites, as revealed by XPS analysis of the pristine carbon surface. All these features highlight that the carbon porosity and surface chemistry are key parameters to design efficient air cathodes for Li-O2 batteries performing at low current density.