The behaviour of lithium electrodes in propylene and ethylene carbonate: Te major factors that influence Li cycling efficiency

The Li cycling efficiency surface chemistry and Li morphology in ethylene carbonate (EC) and propylene carbonate (PC) based electrolyte solutions were investigated and correlated. Surface sensitive ex situ FTIR spectroscopy, X-ray microanalysis and scanning electron microscopy were used in conjunction with standard electrochemical techniques. EC is more reactive than PC in electroreduction processes and is reduced on noble metals to ethylene dicarbonate. The difference in reactivity between the two solvents is discussed, based on MO ab initio calculations of their radical anions (and Li⁺ stabilized radical anions). In spite of the high reactivity of these systems to lithium, the Li cycling efficiency is strongly dependent on the presence of additives and contaminants at the ppm level that modify the Li surface chemistry in solutions. The two alkyl carbonate solvents decompose when stored over activated Al₂O₃ and CO₂ is formed. The presence of CO₂ in solutions increases the Li cycling efficiency considerably due to the formation of Li₂CO₃ on the Li surfaces.