How to understand the reversible capacity decay of the lead dioxide electrode

Unfavourable cycling conditions may successively lead to a porous PbO2 structure with poor mechanical and electrical connection between the particles of which the porous electrode is built. Increasing active material resistance of this skeleton will induce a more and more inhomogeneous current distribution over the active mass, with successive restriction of the discharge/recharge processes to the active material area near to the grid. This leads to excessive mass utilisation in this very region, which aggravates the situation from cycle to cycle. The results are active material softening and shedding. This change of PbO2 structure can be reverted, when cycling is continued with modified regimes. By this and by other means, lost capacity may be recovered. This effect is called reversible capacity decay or reversible insufficient mass utilization (RIMU). The Aggregate-of-Spheres Model (AOS), based on the behaviour of sintered bodies, describes semi-quantitatively the phenomena at the connecting sites of the particles of the PbO2 active material, explaining the experimental findings of reversible capacity decay. The main features of reversible capacity loss are discussed in terms of AOS explanations, in order to work out strategies and limits for the improvement of lead/acid battery performance.