Isothermal Microcalorimetric Analysis of Li/CF_x Battery Discharge

Lithium/carbon monofluoride (Li/CF_x) batteries generate substantial amounts of heat during discharge in part due to the large deviation of the loaded voltage from the measured and thermodynamically predicted open circuit voltage. Here, we further analyze this system by estimating the equilibrium voltage ( V e q ) and its temperature dependence ( d V e q / d T ) over the entire discharge range. Based on these results, the ohmic and entropic heat contributions to the overall heat flow are calculated from experimental data. The ohmic heat flow is consistent with an electrochemical impedance spectroscopy model and electrode density measurements, indicating decreasing porosity of the electrode during discharge. Entropic heating increases during discharge as the cell reaction becomes increasingly entropically unfavorable. The total energy dissipated by the cell (electrical and thermal) remains similar over the entire discharge, evidence for a two-phase reaction with a constant, and rate-independent, Δ G r x n of about −465 kJ mol⁻¹. However, near the end of discharge (>90% DOD), Δ G r x n changes significantly indicating possible secondary reactions.