Influence of Ion Diffusion on the Lithium-Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes-Graphene Substrate.

Lithium-oxygen (Li-O) batteries are nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of these batteries is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li-O battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m g) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of the hydrophobic PTFE polymer on their surface (<20 wt %). The electrochemical characterization of Li-O cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs Li/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs Li/Li. Furthermore, the relatively high impedance of the Li-O cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm (referred to the geometric area of the GDLs). The Li-O battery performances are rationalized by the investigation of a practical Li diffusion coefficient () within the cell configuration adopted herein. The study reveals that is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, values range from ∼10 to ∼10 cm s during the ORR and ∼10 to ∼10 cm s during the OER. The most performing GDL is used as the support for the deposition of a substrate formed by few-layer graphene and multiwalled carbon nanotubes to improve the reaction in a Li-O cell operating with a maximum specific capacity of 1250 mA h g (1 mA h cm) at a current density of 0.33 mA cm. XPS on the electrode tested in our Li-O cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.