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The mechanisms for the 3PbOPbSO4H0O (denoted 3B5) -* PbO, and 4PbOPbSO4 (denoted 4BS) -Pb02 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 Pb02 since the 3BS -Pb02 reaction occurred through a dissolution recrystallization process. In contrast, textural relationships between needle-shaped 4BS and Pb02 were evidenced by SEM analyses. Indeed, 4BS was found to transform into PbSO4, which in turn transformed into PbO., while retaining the needle shape as long the reaction proceeded. The thickness of the Pb09 needle crust surrounding the 4BS needle core was found to depend on both the temperature and soaking duration. A model predicting a decrease in the capacity with the increase in the needle thickness, 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 4B5 cured industrial samples from 10-15 to 3-4 m.

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