Analyses of passivation films on lithium and lithium alloys by electrochemical quartz crystal microbalance

Electrochemical quartz crystal microbalance (EQCM) measurements were carried out during electrochemical deposition/dissolution of Li on the Ti and Pt substrate electrodes of AT-cut quartz crystals in a 1 M LiPF6 in propylene carbonate (PC) and dimethoxyethane (DME) (1:1 in volume) and a 1 M LiPF6 in ethylene carbonate (EC) and DME (1:1 in volume). The mass change during Li deposition on the Pt substrate was about half that on the Ti substrate because the alloy formation of Li–Pt suppressed passivation reactions of deposited Li. Furthermore, it was also suggested that the amount of organic compounds in passivation films formed in the PC-based electrolyte was larger than that in the EC-based electrolyte due to PC reacting more easily with Li than EC. The EQCM measurement of the passivation film formations during alloying Li with Pb electrodeposited on the Ti substrate was also conducted successfully. The Li–Pb alloy formation occurred under 0.5 V, in which the potential profile was divided into two regions associated with the phase diagram of Li–Pb alloy. The mass change due to passivation film formations increased in accordance with the changes of Li–Pb alloy compositions into Li rich phases, which were thought either to enhance the reactiveness of Li or decrease the diffusiveness of Li in Li–Pb alloy.