Li passivation in different electrolytes during storage and cycling - An impedance spectroscopy study

A comparison of the Li passivation was made by impedance spectroscopy in four solutions of interest in Li cells, i.e., LiClO4PC, LiClO4PC/DME, LiAsF6EC/2MeTHF and LiAsF6THF/2MeTHF. after some days of storage, usually three semicircles were observed in the impendance plots. They have been assigned to the geometrical characteristics of the passivating film, to the Rct/Cdl parallel, and to the diffusional impedance (Zd), respectively. The main contribution to the interphase impedance comes by the charge transfer process. Higher Rct were observed in the two solutions where Li can be cycled more efficiently, i.e. LiAsF6EC/2MeTHF and LiAsF6THF/2MeTHF. During an extended cycling in a Li//LiClO4PC/DME//Li1+xV3O8 cell, it was noticed that both Rct and Zd markedly increased with cycle number at the Li electrode. The latter was therefore the one mainly responsible for capacity decline and cell failure. © 1990