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

Lithium-oxygen (Li-O2) batteriesare nowadaysamong the most appealing next-generation energy storage systems inview of a high theoretical capacity and the use of transition-metal-freecathodes. Nevertheless, the practical application of these batteriesis still hindered by limited understanding of the relationships betweencell components and performances. In this work, we investigate a Li-O2 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 hydrophobicPTFE polymer on their surface (<20 wt %). The electrochemical characterizationof Li-O-2 cells using bare GDLs as the support indicatesthat the oxygen reduction reaction (ORR) occurs at potentials below2.8 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 electrochemicalactivation achieved by voltammetry. The Li-O-2 cellsdeliver high reversible capacities, ranging from & SIM;6 to & SIM;8mA h cm(-2) (referred to the geometric area of theGDLs). The Li-O-2 battery performances are rationalizedby the investigation of a practical Li+ diffusion coefficient(D) within the cell configuration adopted herein.The study reveals that D is higher during ORR thanduring OER, with values depending on the characteristics of the GDLand on the cell state of charge. Overall, D valuesrange from & SIM;10(-10) to & SIM;10(-8) cm(2) s(-1) during the ORR and & SIM;10(-17) to & SIM;10(-11) cm(2) s(-1) during the OER. The most performing GDL isused as the support for the deposition of a substrate formed by few-layergraphene and multiwalled carbon nanotubes to improve the reactionin a Li-O-2 cell operating with a maximum specificcapacity of 1250 mA h g(-1) (1 mA h cm(-2)) at a current density of 0.33 mA cm(-2). XPS onthe electrode tested in our Li-O-2 cell setup suggeststhe formation of a stable solid electrolyte interphase at the surfacewhich extends the cycle life.