THE THERMAL-STABILITY OF LITHIUM POLYMER BATTERIES

The electrical. (ionic) conductivity of polymer electrolytes for lithi um-polymer cells increases greatly with increasing temperature (in con trast to the thermal conductivity). This paper examines, by both a Lin ear perturbation analysis and by numerical calculation, an instability that could arise from this temperature dependence. The instability is one where a local perturbation to a higher temperature results in hig her electrical conductivity and therefore the passage of more current in this region with an attendant increase in heat generation. If this excess heat generation cannot be conduced into adjacent cooler regions , instability (growth of the perturbation) results. The numerical calc ulations were carried out for constant potential drop across the elect rolyte, for constant mean current density, and for constant mean power discharge schedules. The calculations suggest that this instability i s unlikely for small batteries (of the order of 0.1 m in the direction s parallel to the electrodes) but may occur for larger batteries such as those intended for use in an electric vehicle. However, the growth rates of the perturbations do not appear to be high. Changes in cell m aterials /thickness have little effect on the instability. The Linear perturbation analysis is in approximate agreement with the numerical c alculations.