CeO2 Nanoparticle-Guided Formation and Decomposition of Amorphous Li2O2 on Carbon Nanotube Cathode in a Li-O2 Cell

Abstract

The structure and morphology of lithium peroxide (Li2O2) in a lithium-oxygen (Li-O2) cell have been widely explored by their correlation to recharge potential. A toroidal Li2O2 crystal with 0.1-1 μm size is a typical and predominant product observed on various carbonaceous cathodes after the 1st discharge (2Li + O2 ↔ Li2O2)1 while the bulk size and poor conductivity of Li2O2 incurs its sluggish decomposition during the recharge, which results in a rapid potential rise more than 4.2 V vs. Li/Li+ concomitant with severe side reactions. This potential shooting can be suppressed by some conductive additives such as metal and metal oxide nanoparticles (NPs), where non-crystalline and/or thin Li2O2 is interestingly observed after discharge. For example, our previous work demonstrated less 4.0 V of potential for the whole recharge process in the presence of RuO2 NPs that led to the formation of amorphous Li2O2 film on the carbon nanotube (CNT) cathode during discharge via its high O2 adsorption affinity.2 Therefore, it is considered that the physical property of Li2O2, arising from both the poor crystallinity having higher ionic and electronic conductivity3 and the thin-film shape providing large contact area with the CNT cathode,4 is beneficial to smooth decomposition of Li2O2 at low potential. However, the criteria for promising additives contributing to the structure and morphology engineering of Li2O2 and mechanistic study for the formation and decomposition of such a unique structure of Li2O2 have not as yet been conducted. Here we present in-depth study of nucleation, growth and decomposition process of non-crystalline Li2O2 in the presence of cerium oxide (CeO2) NPs using TEM and correlate this to Li-O2 cell performance. The small diameter of CeO2 NP (~5 nm) has strong affinity for the oxygen species (O2, O2 - and O2 2-),5-6 which is accordingly a good candidate to investigate the evolution of Li2O2 morphology during discharge and recharge. At initial discharge, the nucleation of Li2O2 appears on the CeO2 NPs, but not on the CNT, from which the Li2O2 laterally grows and coats over the CNT surface. The CeO2 NPs determine the height of Li2O2 film, thus thicker Li2O2 film can be observed in the area of agglomerative CeO2 NPs. Further discharge allows a vertical growth (i.e. thicker film) once the thin Li2O2 film covers the entire CNT surface. There is no evidence of Li2O2 crystal structure, which is very distinguished from the toroidal and polycrystalline Li2O2 observed on a CeO2-free CNT (Fig. 1). The decomposition of Li2O2 seems to emerge from the amorphous and thin Li2O2 surface, which results in low recharge potential at the initial stage. However, the CeO2 NPs having strong O2 and CO2 affinity7 and poor conductivity become a hurdle for the completion of Li2O2 decomposition at low potential unlike the RuO2 NPs. The implication of CeO2 role and characteristics of the unique structure of Li2O2 based on different discharge and recharge status will be discussed in details in the presentation.