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Mechanism of Thermal Runaway in VRLA Batteries and Methods to Suppress It - Phase II

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Final rept.

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The aim of this project is to elucidate the mechanism of thermal runaway in VRLA batteries and especially the eventual role of the AGM separator. The oxygen evolution OER and oxygen recombination reactions ORR proceed in the cells, and the only work done by the input power is to transfer water from anode to cathode through these reactions, the remaining energy being dissipated as heat. The heat generators in the cell are the electrochemical reactions through their activation overpotentials, the exothermic chemical reactions and the exothermicity of ORR plus Joule Ohmic heating in electrolyte and solid phases. We have shown that, within the range of currents used up to 1C, the major contribution comes from electrochemical reactions. The cell temperature is at most in a first order dependence of recombinant current. The OER at constant applied voltage and fixed ambient temperature is accelerated by the rise of internal cell temperature and overpotential, as the negative plate is depolarized by ORR due to enhanced separator permeability for oxygen flow by electrolyte displacement andor drying and negative plate local desaturation. If the negative plate potential is stabilized at PbPbSO4 potential and cannot be shifted by any current applied, the cell behaviour is controlled solely by OER reaction. Thus, a lower applied voltage is enough to produce a current sufficient to generate conditions for thermal runaway. The results obtained in this study suggest strongly that by modification of separator properties it is possible to achieve better thermal control, in concert with other cell design improvements, plus the usual external measures as power input management and passiveactive cooling.

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  • Electrochemical Energy Storage

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