Cycle performance of Li-O-2 cells with N-methyl-N-propylpiperidinium bis(trifluoromethansulfony) imide (PP13TFSI)-LiClO4 electrolytes and vertically aligned carbon nanotube (VACNT) cathodes has been investigated. It is found that both the current density and the depth of discharge have great influence on the capacity utilization, voltage polarization and cycle life. Abacus-ball-shaped Li2O2 particles produced at discharge are identified, and their complete decomposition at charge is confirmed. Meanwhile, formation of Li carbonates around the VACNTs is detected after the first cycle. The carbonates accumulate and gradually become predominant over Li2O2 upon cycling, leading to decrease of the discharge capacity and increase of the cell resistance. These results indicate that reduction of the carbonates formed around the VACNTs is of critical importance for improving cycle performance of the cells investigated here. (C) 2013 Elsevier B.V. All rights reserved.