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

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 1^st 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.

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Fig. 1 TEM images of the discharged (a) CNT and (b) CeO₂/CNT electrodes. Both Li-O₂ cells are discharged at 50 mV g^-1_-CNT to 2.2 V vs. Li⁺/Li, using 0.5 M LiClO₄ in TEGDME as electrolyte.