Combining in situ electrochemistry, operando XRD & Raman spectroscopy, and density functional theory to investigate the fundamentals of Li₂CO₃ formation in supercapacitors

Higher voltage aqueous electrolytes in supercapacitors are a promising technology in energy storage applications due to their high power, low cost, and environmental friendliness. However, applying voltages under abusive conditions would cause damage or failure to the cell. Here, we report the transient modification of the electrode & electrolyte interface tracked by operando synchrotron X-ray diffraction and Raman spectroscopy analysis combined with in situ electrochemistry and theoretical calculations to explore the formation of lithium carbonate species and reversible degradation in supercapacitors. Symmetrical electrochemical supercapacitors were prepared with nickel oxide (NiO) decorated multiwalled carbon nanotube (MWCNT) electrodes and filled with 1.0 mol L-1 Li2SO4 aqueous electrolyte. Operando XRD analysis of NiO@MWCNT shows crystalline Li2CO3 formation when applying higher operating cell voltages, close to 2.0 V, in anodic polarization and decomposition in the cathodic polarization, evidencing the reversibility of the system. Studies by operando Raman spectroscopy showed that the carbon electrodes oxidise with structural changes on the carbon electrode surface. A theoretical analysis is presented to relate the effect of Li2CO3 formation on the electronic properties. Li2CO3 formation deformed the electron localization function of nanotubes, suggesting that Li-ion diffusion can be restricted, influencing Li2CO3 formation and reversible decomposition. A plausible reason for Li2CO3 formation is CO2 evolution due to the degradation of the MWCNT electrode and Li+ ions from the electrolyte, catalysed by NiO nanoparticles. Interestingly, Li2CO3 formation is reversible on the full cycle cell scan in the presence of NiO, being able to explore different applications in energy storage.