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

Lithium-oxygen (Li-O 2 ) 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 2 battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m 2 g -1 ) 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 2 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 2 cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O 2 cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm -2 (referred to the geometric area of the GDLs). The Li-O 2 battery performances are rationalized by the investigation of a practical Li + diffusion coefficient ( D ) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, D values range from ∼10 -10 to ∼10 -8 cm 2 s -1 during the ORR and ∼10 -17 to ∼10 -11 cm 2 s -1 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 2 cell operating with a maximum specific capacity of 1250 mA h g -1 (1 mA h cm -2 ) at a current density of 0.33 mA cm -2 . XPS on the electrode tested in our Li-O 2 cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.