Study of the softening of the positive active-mass in valve-regulated lead-acid batteries for elect fic-vehicle applications

摘要

Valve-regulated lead-acid (VRLA) batteries have been proposed as energy sources for electric vehicles because of their good power performance and low price. Unfortunately, however, intensive utilization of the positive active-mass causes softening of this material and thereby, reduces battery cycle-life. Experimental cells have been developed and used in order to understand the mechanism of softening. The evolution kinetics of the β-PbO_2 has been studied with very dynamic discharge profiles: different steps of the β-PbO_2 life have been explored. The experimental results have been compared and correlated with positive active--material used in electric vehic1es (field tests). Scanning electron microscopy reveals a continuous growth of the β-PbO_2 microstructure throughout the cycle-life of the active mass (from ~ l to ~ 2 μm). This is confirmed by a decrease in the specific surface area (from 4 to 2.5 m~2 g~-1). Powder X--ray diffraction reveals similar changes in the nanostructure (from ~ 20 to 90 nm): the crVstaI1ites, as microstructure particles, grow with the number of cycles. These results also show the difference between the active material bound to the electrode and the β-PbO_2 crystallites which have lost contact with the plate; the latter are larger than the former (~ 100 nm vs. ~ 80 nm). The investigations confirm that the capacity loss of VRLA batteries is strongly linked to a micro- and nano-textural evolution of the β-PbO_2 This finding supports the theory that there is a progressive expansion of the positive active-material, which leads to a decrease in the contact zones until final shedding of the material takes place. Strategies to overcome this effect have been tested and it appears that electrolyte additives and electrical treatments could be useful in extending battery cycle-life.