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

Lithium-oxygen (Li-O₂) 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-O₂ battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m² 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-O₂ 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-O₂ cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O₂ cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm^-2 (referred to the geometric area of the GDLs). The Li-O₂ 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 cm² s^-1 during the ORR and ∼10^-17 to ∼10^-11 cm² 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-O₂ 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-O₂ cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.