Optimization of the positive active material capacity in lead-acid cells through control of the basic lead sulfate precursor

The mechanisms for the 3PbO . PbSO4. H2O (denoted 3BS) --> PbO2 and 4PbO . PbSO4 (denoted 4BS) --> PbO2 transformations were studied from well-defined, pure samples formed in a laboratory cycling test cell. The oxidation reactions were observed using X-ray diffraction and scanning electron microscopy (SEM). No structural memory effect was found between 3BS and PbO2 since the 3BS --> PbO2 reaction occurred through a dissolution recrystallization process. In contrast, textural relationships between needle-shaped 4BS and PbO2 were evidenced by SEM analyses. Indeed, 4BS was found to transform into PbSO4, which in turn trans-formed into PbO2 while retaining the needle shape as long the reaction proceeded. The thickness of the PbO2 needle crust surrounding the 4BS needle core was found to depend an both the temperature and soaking duration. A model predicting a decrease in the capacity with the increase in the needle thick, ness, as a consequence of the inactive 4BS presence inside the needles, is proposed. The ability to prepare samples of controlled morphology revealed a decrease in capacity with the increase in the needle thickness, thereby confirming the model prediction. But more importantly we showed that a 20-25% increase in the positive electrode efficiency could be achieved by reducing the thickness of the usual 4BS cured industrial samples from 10-15 to 3-4 mu m.