Exploring carbon electrode parameters in Li-O₂ cells: Li₂O₂ and Li₂CO₃ formation

Ensuring the stability of the electrode and electrolyte in Li-O-2 batteries and achieving a comprehensive understanding of parasitic side reaction management during cycling are key issues for the progress of this promising energy storage technology. Conditions that favour formation of either Li2O2 or Li2CO3 in Li-O-2 cells on carbon-based electrodes were investigated. Operando Raman microscopy measurements and ex situ Raman and X-ray photoelectron spectroscopy (XPS) analyses were performed for Li-O-2 systems using Li[ClO4]/DMSO as the electrolyte and carbon paper (CP) and carbon paper with carbon nanotubes (CPCNT) as electrodes. Using CP electrodes (either treated or untreated with O-2 plasma), the major discharge product formed was Li2O2. In contrast, for CPCNT electrodes, the formation of Li2CO3 as the main discharge product was observed at lower capacities, then significant formation of Li2O2 proceeded at higher discharge capacities. XPS highlighted that the surface chemistry of the CPCNT electrode comprised fluorine and a variety of iron species, which could be linked to the promotion of Li2CO3 formation. Furthermore, it was observed that when Li2CO3 is the main discharge product, the active sites of functional groups on carbon surfaces that favour carbonate formation become coated/passivated. Consequently, the dominant reaction pathway then alters, leading to the growth of Li2O2 over the surface. These outcomes emphasized the important role in cycling stability of the active sites on carbon electrodes, arising from the synthesis process or possible contaminants.