Thermopassivation of positive battery plates

After formation, the positive plates of lead acid batteries are washed and dried. When they are dried at temperature above 70^oC in a drying oven they lose part of their energy. This phenomenon is called thermopassivation. Thermopassivation is measured by the difference between the galvanostatic discharge potential curves of the thermopassivated and unpassivated positive plates. The dependence of thermopassivation on temperature of drying, time of stay of the plates in H₂SO₄ solution, current density, and number of charge-discharge cycles has been studied.

Processes causing thermopassivation and depassivation. Thermopassivation is a result of a solid-state reaction between Pb grid and PbO₂ corrosion layer at t^o>70^oC, which leads to formation of nonstoichiometric PbO_n layer (n<1.5) at the grid/PAM interface. This oxide layer is a semiconductor with high resistance which depends on the value of n. This layer is responsible for the high polarization of the positive plates during discharge and charge. At high charge currents, O₂ is evolved at the positive plates. It penetrates into the corrosion layer and oxidizes the PbO_n layer to PbO₂. Consequently, thermopassivation stops.

References

1. D. Pavlov, St. Ruevski, Thermopassivation of the lead dioxide plate of lead-acid batteries, J. Electrochem. Soc., 126 (1979) 1100
2. D. Pavlov, St. Ruevski, Processes at Thermopassivation of the Lead Dioxide Plate of the Lead-Acid Batteries, 28 Meeting ISE, Electrochemical Power Sources, Extended Abstracts, p.97, Varna, Bulgaria, 1977
3. D. Pavlov, St. Ruevski, Thermopassivation of lead-acid battery plates, 29 Meeting of ISE, Part II, Extended Abstracts, p.836, Budapest, Hungary, 1978

Keywords: thermopassivation of positive plates