(388c) Understanding the Formation and Evolution of Lead Sulfate Films at the Electrode/Electrolyte Interfaces in a Lead-Acid Battery

Lead-acid batteries have been a major part of the economy for over 100 years. Recently, they have seen significant use in next-generation energy storage applications due to their high energy efficiency (up to 80%), low capital cost, and the existence of a strong manufacturing, distribution, and recycling infrastructure (up to 96% of the lead from spent batteries is currently recycled). Despite these advantages, lead-acid batteries have seen limited use in hybrid-electric vehicles due to a low cycle life in comparison to lithium ion batteries. The low cycle life is a result of the progressive build-up of an irreversible, inactive lead sulfate film on the surface of the negative electrode during operation of the battery. Over time, the loss in surface area due to accumulation of the film is too great for the battery to deliver the high reaction rates required by engine-cranking, acceleration, and regenerative braking in hybrid-electric vehicles. This work seeks to address this issue by investigating the mechanisms of formation of the lead sulfate film in order to understand how it progresses from an active to an inactive state. In particular, we attempt to determine the key structural features of the film which are responsible for making it irreversible. The end goal is to identify methods for controlling film structure in order to prevent the formation of the inactive state at the electrode/electrolyte interface.

In this study, the formation and initial structure of the lead sulfate film is investigated in-situ with a special, in-house designed, micro-electrochemical cell using transmission x-ray microscopy (TXM) available at the X8C beamline of the National Synchrotron Light Source at Brookhaven National Laboratory. This nanoimaging technique is capable of producing 40 x 40 μm micrographs at one second time intervals with a sub-40 nm resolution. For these experiments, an electrical bias is applied to a thin lead sample in sulfuric acid, and the electrochemical formation (and dissolution) of the lead sulfate film at the electrode/electrolyte interface is observed, in real-time, using TXM. The resulting images yield crucial information about the structure of the film and the kinetics of formation, such as: nucleation site density, crystal size at passivation, and induction time for film growth. In conjunction with the TXM experiments, extended charge/discharge cycling of lead in sulfuric acid has been performed. The cycling is coupled with periodic electrochemical impedance spectroscopy measurements to track the changes in the impedance of the lead sulfate film, which is grown and reduced during cycling. Variations in impedance are associated with the evolution of the film structure as it progresses to the inactive state. These results are coupled with the structural and kinetics observations from the TXM to provide further insight into the mechanisms of film evolution. The combination of the two experiments yields an important understanding of how the lead sulfate film forms and progresses in a lead-acid battery.