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 beta -PbO2 has been studied with very dynamic discharge profiles: different steps of the beta -PbO2 life have been explored. Scanning electron microscopy reveals a continuous growth of the beta -PbO2 microstructure throughout the cycle-life of the active mass, confirmed by a decrease in the specific surface area. Powder X-ray diffraction reveals similar changes in the nanostructure: the crystallites, 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 beta -PbO2 crystallites which have lost contact with the plate; the latter are larger than the former. The investigations confirm that the capacity loss of VRLA batteries is strongly linked to a micro- and nano-textural evolution of the beta -PbO2. 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.
Study of the softening of the positive active-mass in valve-regulated lead-acid batteries for electric-vehicle applications
1999
10 Seiten, 23 Quellen
Aufsatz (Konferenz)
Englisch
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