Structural Properties of the PbO2 Active Mass Determining Its Capacity and the “Breathing” of the Positive Plate during Cycling

By measuring the distribution of the volume and surface area of the pores in positive active masses (PAM's) having various structures, it was found that depending on their size the pores perform two functions: the macropores build up the transportation systems, along which the fluxes of ions taking part in the reaction flow, and the micropores, which provide the active surface, onto which the reactions occur. It was established that the plate capacity is determined by the volume of the transport pores and depends also on the surface area of the active mass. During discharge both the volume and the surface area of the pores decrease, while the plate thickness grows. This is related to the newly formed nonequilibrium crystals, which generate internal stresses tending to expand the skeleton of the active mass. When discharged plates are kept for a longer period at the open‐circuit potential, the crystals recrystallize, this resulting in a contraction of the plate. During charge the active mass continues to shrink but the initial thickness of the plate cannot be restored in all cases. This "breathing" of the plate during discharge and charge becomes more and more pronounced as the number of cycles grow. This process is associated with the deterioration of the mechanical integrity of the skeleton structure. The "breathing" brings about an irreversible swelling of the active material, deterioration of the contacts between the agglomerates, and decay of the plate capacity. The advancement of this process results finally in the ultimate failure of the positive plate of the lead acid battery.