Chemical and Morphological Changes of Li-O2 Battery Electrodes upon Cycling

We report considerable chemical and morphological changes of reaction products in binder-free, vertically aligned carbon nanotube (VACNT) electrodes during Li-O-2 battery cycling with a 1,2-dimethoxyethane (DME)-based electrolyte. X-ray absorption near edge structure (XANES) of discharged oxygen electrodes shows direct evidence for the formation of Li2CO3-like species at the interface between VACNTs and Li2O2 but not significantly on the Li2O2 surfaces exposed to the electrolyte. Although Li2O2 and Li2CO3-like species were largely removed upon first charge, the oxidation kinetics became increasingly difficult during Li-O-2 cycling, which is accompanied by the accumulation of Li2CO3 in the discharged and charged electrodes as evidenced by selected area electron diffraction (SAED) and transmission electron microscopy (TEM). Together, these results indicate that the irreversibility during Li-O-2 cycling in DME can be attributed largely to the growth of Li2CO3-like species associated with the reactivity between carbon and Li2O2 or other reaction intermediates.