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

Abstract

Ensuring the stability of the electrode and electrolyte in Li–O₂ 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–O₂ 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–O₂ 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 O₂ 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.