Conversion of tetrabasic lead sulfate to lead dioxide in lead/acid battery plates

The formation of cured lead/acid battery plates containing a high level (∼ 70 wt.%) of tetrabasic lead sulfate (4PbO·PbSO44BS) has been studied under both cyclic voltammetric and constant-potential conditions. The resulting phase composition is determined by an X-ray diffraction technique, while the morphology is examined by both optical and scanning-electron microscopy. The cyclic voltammogram for a 4BS cured plate displays only one anodic peak in the ‘PbO2’ region. X-ray diffraction phase-analysis confirms that this is due to the conversion of PbSO4 to the β polymorph of PbO2. Thus, the long-held belief that 4BS promotes the formation of α-PbO2 is shown to be incorrect. The formation process commences at the grid surface and then spreads into the cured material, preferentially via the surface of the 4BS crystalline network. The product consists of a large number of fine β-PbO2 particles gathered into porous agglomerates that retain the overall geometry of the 4BS network. The dioxide morphology can vary widely — from laminar to anhedral shapes — and is determined largely by the value of the applied potential. Thus, the industrial practice of conducting formation under constant-current (as opposed to constant-potential) conditions may result in plates with inconsistent physicochemical characteristics.