Li+ ion exchange in CeO2–TiO2 sol–gel layers studied by electrochemical quartz crystal microbalance

The paper reports first on the electrochemical behavior in liquid Li+ electrolytes of 200 nm thick single sol–gel (CeO2)0.81–TiO2 electrochromic (EC) layers deposited by the dip-coating process. The electrolytes were solutions of 1 M LiClO4 dissolved in dry propylene carbonate (PC) (containing 0.03 wt% of water) and wet PC containing up to 10 wt% of water, respectively. Then an electrochemical quartz crystal microbalance was used as a sensitive detector to analyze the mass changes occurring during the Li+ ion exchange processes. These electrochemical processes were studied for 370 nm thick double layers, deposited on gold-coated quartz crystal electrodes and sintered at 450 °C in air. The electrolytes were the same solutions with water content varying from 0.03 up to 3 wt% of water. The processes have been studied in the potential range from −2.0 to +1.0 V vs. Ag/AgClO4 during 100 voltammetry cycles. The composition of the (CeO2)0.81–TiO2 layers was found to change during the early cycles, mainly because of an irreversible Li+ intercalation. It was found, however, that the mass change observed during cycling is not due only to a pure Li+ ion exchange process but also involves the adsorption/desorption or exchange of other cations and anions contained in the electrolyte. These ions are Li+ and ClO4− in dry electrolyte and Li+, hydrated Li(H2O)n+ and ClO4− in wet electrolyte. The improvement of the reversibility of the intercalation and deintercalation processes as well as the faster kinetics observed in wet electrolytes are finally discussed in terms of a model in which the formation of hydrated Li+ ions takes an important role.