Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with CeO2 Nanoparticles in Li-O2 Batteries.

In lithium-oxygen (Li-O2) batteries, it is believed that lithium peroxide (Li2O2) electrochemically forms thin films with thicknesses less than 10 nm resulting in capacity restrictions due to limitations in charge transport. Here we show unexpected Li2O2 film growth with thicknesses of ∼60 nm on a three-dimensional carbon nanotube (CNT) electrode incorporated with cerium dioxide (ceria) nanoparticles (CeO2 NPs). The CeO2 NPs favor Li2O2 surface nucleation owing to their strong binding toward reactive oxygen species (e.g., O2 and LiO2). The subsequent film growth results in thicknesses of ∼40 nm (at cutoff potential of 2.2 V vs Li/Li(+)), which further increases up to ∼60 nm with the addition of trace amounts of H2O that enhances the solution free energy. This suggests the involvement of solvated superoxide species (LiO2(sol)) that precipitates on the existing Li2O2 films to form thicker films via disproportionation. By comparing toroidal Li2O2 formed solely from LiO2(sol), the thick Li2O2 films formed from surface-mediated nucleation/thin-film growth following by LiO2(sol) deposition provides the benefits of higher reversibility and rapid surface decomposition during recharge.