Aqueous Li-Air Battery Cathode Using Carbon Nanotube Arrays

Abstract

Li-air batteries (LABs) have attracted great interest due to their high theoretical energy density and several aqueous-organic hybrid electrolyte based Li-air battery (HyLAB) systems have been proposed.1-5The cathode structure for a LAB is critical to its performance, as oxygen from ambient air must be able to diffuse into the cathode. The cathode needs to be highly porous and reactive to achieve large oxygen flux and high power operation.In this work, we demonstrated a cathode structure with sparsely populated, vertically aligned carbon nanotube arrays (CNTAs). The CNTAs are nitrogen doped with dislocated-graphene stacking grown directly on carbon fiber papers and investigated as hierarchical air cathodes. These metal-free CNTAs were obtained by plasma enhanced chemical vapor deposition (PECVD) on a backing carbon paper, without further electrode processing.The sparse CNTAs not only provide effective pathways for reacting species, but also show significantly high catalytic activity, which is comparable to a 20% Pt/C electrocatalyst. The high activity of the CNTAs is attributed to the rich graphene edges exposed on the CNT surface and nitrogen doping. Hybrid Li-air batteries with such nanostructured cathode show a consistent discharging capacity of 710 mAh/g and a specific energy of 2057 Wh/kg at 0.5 mA/cm2 in alkaline catholytes. Stable charge-discharge cycling at 0.5 mA/cm2 showed an average potential difference of 1.35 V, indicative of relatively small overpotential and high round trip efficiency. Furthermore, the hybrid Li-air battery based on the hierarchical cathode can reach a power density over 100 W/m2.