Study of deposition and dissolution processes of lithium in carbonate-based solutions by means of the quartz-crystal microbalance

The quartz crystal microbalance (QCM) was used to investigate the formation of surface films on a lithium anode. Whereas few studies have dealt directly with electrode mass changes, QCM was employed here to investigate in situ the deposition and dissolution of lithium on a Ni substrate, with respect to mass changes and changes in the surface-film morphology. The formation and destruction of the deposited surface film was followed by a real-time measurement of the resonance frequency (Delta f) and the resistance parameter (Delta R). Delta R expresses the energy loss to the oscillation of the crystal at resonance state, which gave us useful information on the variation of surface roughness during charge-discharge cycles. The Delta f and Delta R variations are discussed for three different electrolyte systems, viz, LiClO4, LiCF3SO3, and LiPF6 in propylene carbonate or diethyl carbonate solutions. Upon cycling, Delta f showed an irreversible decrease indicating continuous formation and destruction of the surface film. The extent of Delta f shift depended strongly on the electrolyte being larger for LiClO4/PC and LiCF3SO3/PC systems than that for LiPF6/PC. The Delta R measurement indicated a smoother and more uniform surface of the deposited lithium for LiPF6/PC compared to those for LiClO4/PC and LiCF3SO3/PC. In the LiPF6/PC system, the mass was found to increase even during the initial dissolution process. Such behavior can be explained by a healing process involving the subsequent chemical formation of a surface film right after dissolution of lithium. This process is faster than the lithium dissolution process itself in this case.