Progressive changes in positive active material over the lifetime of a lead–acid battery

Abstract

Time-of-flight neutron diffraction for a series of lead–acid battery positive plate samples has shown that β-PbO 2 is not stoichiometric, but has a deficiency of Pb relative to oxygen giving Pb 0.92O 2 to Pb 0.96O 2. The Pb vacancies cause a charge deficiency that is balanced by crystallographic hydrogen on a disordered site. The position of hydrogen is typical of hydrogen bonding as has been previously proposed based on nuclear magnetic resonance spectra. With either rapid or conventional charging, the amount of hydrogen substitution decreases and the β-PbO 2 tends toward a stoichiometric composition at battery failure. High resolution SEM imaging of these and other battery samples shows a progressive change in the morphology of β-PbO 2 and these changes appear to be the same for both rapid or conventional charging. The morphology progresses from elongated, well-formed crystals to equidimensional crystals to dense clusters of crystals with a consequent reduction in surface area. Neither stoichiometry nor morphology changes necessarily contribute to battery failure, but both progressively change over the cycle life of the battery.