Structurally Tuning Li2O2 by Controlling the Surface Properties of Carbon Electrodes: Implications for Li-O2 Batteries

In lithium oxygen (Li-O-2) batteries, controlling the structure of lithium peroxide (Li2O2) can reduce the large overpotential of the charge process as this affects the ionic and electronic conductivities of Li2O2. We demonstrate, for the first time, the in situ structural tuning of Li2O2 during the discharge process by virtue of the surface properties of carbon nanotube electrodes. We tailored carbon nanotube surfaces to decouple oxygen functional groups, defective edges, and graphitization, which directly influence the surface-binding affinity of O-2 and LiO2. Consequently, conformal and completely amorphous Li2O2 films form in the presence of oxygen functional groups, which can facilely decompose in the subsequent charge. In contrast, crystalline Li2O2 particles grow in more ordered carbon electrodes and consequently require higher overpotential for decomposition. Our comprehensive study reveals the possibility of facile decomposition of Li2O2 by the surface engineering of carbon electrode and gives insights into the parameters to improve Li-O-2 cell performance without any additional promoters such as nanoparticles or soluble redox mediators. In all, this work provides improved understanding of the general role of carbonaceous electrode surfaces toward the enhancement of discharge capacity, charge potential, and stability.