Calorimetric Determination of the Thermoneutral Potential of Li/BCX and Li / SOCl2 Cells

Through a continuous recording of the cell voltage, heat flow, and current, the effective thermoneutral potential, Eetp of Li/BCX and Li/SOCl2 cells were determined in the temperature range, 0-60°C. The depth of discharge (DOD), temperature (T), and cell type (cell chemistry) affect the effective thermoneutral potential. The effective thermoneutral potential, Eetp differs from the classical thermoneutral potential of a cell because it takes into account the heat flow due to nonfaradaic processes. The average effective thermoneutral potential at 25°C (determined by selecting the most constant region of Eetp vs. time of discharge) was 4.0 and 3.84 V for BCX and Li/SOCl2 cells, respectively. Based on the classical approach, the reversible cell potential, Er and temperature dependence of reversible cell potential, dEr/dT for BCX cell were 3.74 V and -0.852 mV/K, respectively, and for Li/SOCl2, Er = 3.67 V and dEr/dT = -0.567 mV/K. The thermal polarization (Eetp - El), where El is the load voltage, for both cells, showed that they are most thermally efficient near 40°C. A measure of the heat flow for both cells at 50% DOD supports this observation. An overall reaction proposed for the BCX chemistry is supported by the calculated thermodynamic parameters.