The electrochemical quartz crystal microbalance as a means for studying the reactivity of Cu2O toward lithium

Transition-metal oxides (MxOy where M is Co, Ni, Cu, or Fe) were recently reported to reversibly react with Li through a mechanism that differs from the classical Li insertion/deinsertion or Li-alloying ones. We report on the peculiar electrochemical reactivity of copper oxide (Cu2O) toward lithium, as deduced from the use of an electrochemical quartz crystal microbalance (EQCM). First, the electrodeposition parameters (current, pH, temperature,...) to prepare single-phase, homogeneous, and weight-controlled Cu2O deposits on a Ti-plated quartz crystal are reported. Then, an electrochemical cell using the Cu2O deposit as the positive electrode and Li as the negative electrode was cycled over the 3-0.02 V range, and the Cu2O weight evolution was monitored. We show that the weight vs. the number of reacted Li+ curve does not change smoothly, but exhibits different sloping weight regimes that were broadly linked to the different electrochemical processes (reduction of Cu2O into Cu plus growth of an organic layer) already mentioned. Therefore, we show that these processes do not occur separately but in conjunction, highlighting the positive attributes of the EQCM technique. (C) 2002 The Electrochemical Society.