Isothermal Microcalorimetric Analysis of Li/CF_x Battery Discharge

Abstract

Lithium/carbon monofluoride (Li/CFx) 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_eq) and its temperature dependence (〖dV〗_eq⁄dT) 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, 〖∆G〗_rxn of about -465 kJ mol-1. However, near the end of discharge (>90% DOD), 〖∆G〗_rxn changes significantly indicating possible secondary reactions.

Isothermal Microcalorimetry Analysis of Li/β-MnO2 Discharge

“…The entropy coefficient dV eq /dT was determined in the same way as earlier works, by the "slope" method. 27,30,31 Briefly, cells were discharged at 40 mA g −1 to a given lithiation state, where the following steps were performed: (1) decrease temperature from 30 to 15 °C at 0.5 °C min −1 and equilibrate for 30 min, (2) increase temperature to 25 °C at 0.5 °C min −1 and equilibrate for 30 min, (3) increase temperature back to 30 °C, (4) discharge to the next lithiation state, and rest 22 h before repeating the temperature profile. The cell voltage and temperature were recorded during this process to obtain the quantities dV/dt (where t is time) and dT/dt, where the quotient yields the desired dV/dT.…”

Elucidating the Role of Electrochemically Formed LiF in Discharge and Aging of Li-CFx Batteries

“…Despite the very high specific energy density of Li-CF x batteries, much remains to be understood about their electrochemical discharge mechanism. Several discharge mechanisms have been proposed, from the direct formation of LiF to the formation of an intermediate compound structure. ,,,− In 1975, Whittingham et al first proposed a two-phase reaction mechanism in which lithium ions intercalate between the fluorinated graphite sheets forming a nonstoichiometric ternary compound (CLi x F). After discharge, X-ray diffraction (XRD) measurements of the cathode structure demonstrated a 3.55 Å increase in the interlayer spacing.…”

Quantifying the Entropy and Enthalpy of Insertion Materials for Battery Applications Via the Multi-Species, Multi-Reaction Model

“…Currently, the entropy coefficient for a lithium-ion battery is measured using a variety of methods (HTFDA, 1 calorimetric, [38][39][40] potentiometric [41][42][43] ) and is typically considered to be a defined material property of the battery cell used as an input to a system level thermal simulation. While this is a robust approach if a battery cell exists and can be tested, determining the entropy coefficient of a battery cell that has not been prototyped yet, and therefore cannot be tested, is critical to consider the reversible heat 44,45 as part of a robust design strategy, 46 particularly when the ratio between active material capacities 47 is a cell design parameter that is in scope for a battery design engineer.…”