2002 Volume 66 Issue 6 Pages 670-675
In order to investigate the discharge/charge process of the active materials for the positive electrode in the lead-acid battery, a PbO2 thin film was deposited on a piezoelectric quartz crystal by reactive sputtering, and then the in situ observations of the mass change in the PbO2 electrode were carried out using the electrochemical QCM technique. The PbO2 thin film produced by sputtering a pure Pb target in an oxygen plasma was identified as a mixture of α-PbO2 and β-PbO2. The discharge process by a very low current suggested that the reaction of PbO2→PbSO4 proceeded first and then the reactions such as PbO2→PbO, PbO→Pb and PbSO4→Pb occurred one after another. During the potentiostatic discharge/charge process, most of the discharge reaction of PbO2→PbSO4 was completed at the beginning of the process, while the charge reaction of PbSO4→PbO2 slowly proceeded. The conversion ratio of the PbSO4 back to the original PbO2 during charge was estimated to be only 52% or so based on the mass change. Right after the start of both the discharge and charge, the mass decreases in sharp intervals as the result of the dissolution-precipitation mechanism were confirmed by the electrochemical QCM technique for the first time. The repetition of the discharge/charge increased the ratio of β-PbO2 to α-PbO2 in the charged product. Moreover, there were PbSO4 crystals of various sizes on the PbO2 surface after discharge, and the small crystals in the lower part dissolved earlier than the large ones to be transformed into the PbO2 particles of tens of nanometers during the charge. These results suggest that the refinement of the PbSO4 crystals during discharge is an important factor to improve the charge of the active materials for the positive electrode.
