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 4P bO . PbSO4 (denoted 4BS) --> PbO2 transformations were studied from we ll-defined, pure samples formed in a laboratory cycling test cell. The oxidation reactions were observed using X-ray diffraction and scannin g electron microscopy (SEM). No structural memory effect was found bet ween 3BS and PbO2 since the 3BS --> PbO2 reaction occurred through a d issolution recrystallization process. In contrast, textural relationsh ips between needle-shaped 4BS and PbO2 were evidenced by SEM analyses. Indeed, 4BS was found to transform into PbSO4, which in turn trans-fo rmed into PbO2 while retaining the needle shape as long the reaction p roceeded. The thickness of the PbO2 needle crust surrounding the 4BS n eedle core was found to depend an both the temperature and soaking dur ation. A model predicting a decrease in the capacity with the increase in the needle thick, ness, as a consequence of the inactive 4BS prese nce inside the needles, is proposed. The ability to prepare samples of controlled morphology revealed a decrease in capacity with the increa se in the needle thickness, thereby confirming the model prediction. B ut more importantly we showed that a 20-25% increase in the positive e lectrode efficiency could be achieved by reducing the thickness of the usual 4BS cured industrial samples from 10-15 to 3-4 mu m.