Analysis of a Lithium/Thionyl Chloride Battery under Moderate‐Rate Discharge

The lithium/thionyl chloride battery (Li/SOCl 2) has received considerable attention as a primary energy source due to its high energy density, high operating cell voltage, voltage stability over 95% of the discharge, large operating temperature range ( 255 to 708C), long storage life, and low cost of materials. 1,2 However, a loss in performance may occur after periods of prolonged storage at high and low temperatures or when exposed to intermittent use. This loss in performance may result in reduced capacity or even worse, catastrophic failure, especially when operated at high discharge rates. High discharge rates and high temperatures promote thermal runaway, which can result in the venting of toxic gases and explosion. 2 Mathematical models can be used to tailor a battery design to a specific application, perform accelerated testing, and reduce the amount of experimental data required to yield efficient, yet safe cells. Models can also be used in conjunction with the experimental data for parameter estimation and to obtain insights into the fundamental processes occurring in the battery. Previous investigators 3,4 presented a one-dimensional mathematical model of the Li/SOCl2 battery. They used porous electrode theory 5 to model the porous cathode and concentrated solution theory 6 for the electrolyte solution to study the effect of various design and operational parameters on the discharge curves. The model equations were written under the assumption that the excess electrolyte was in a reservoir between the separator and the porous cathode. The result was that the electrolyte replenished the porous cathode through the front face of the electrode. The theoretical results showed similar qualitative trends to those observed experimentally. However, a lack of experimental data and unknown values for many of the kinetic and transport parameters as a function of temperature prevented quantitative comparisons. Evans and White 7 presented a parameter estimation technique and used it in conjunction with the one-dimensional mathematical model presented earlier. 4 However, the comparison between simulated and experimental discharge curves was done only for partially discharged cells at ambient temperature. This paper presents a one-dimensional mathematical model for the Li/SOCl2 cell, with model equations similar to those presented previously.3,4 The exception is the modification to the material balance in the porous cathode that accounts for electrolyte replenishment through the top rather than the front of the porous cathode. 8 The model is used to predict discharge curves at low-to-moderate discharge rates (discharge loads #10 V, corresponding to current densities less than 2 mA/cm 2 for a D-size cell). Previous thermal models of Li/SOCl2 cells have shown that under these operating conditions thermal runaway is not a problem. 9,10 Therefore, it is also assumed here that the temperature of the cell is uniform throughout but allowed to change during discharge. 3,4 The model is then used in conjunction with experimental data to obtain estimates for the transference number, diffusion coefficient, and kinetic parameters for the reactions at the anode and cathode as a function of temperature. Using the estimated parameters, the model predictions show good agreement with the experimental data over a wide temperature (255 to 498C) and load range (10 to 250 V). Finally, the model is used to study the effect of cathode thickness on cell performance as a function of operating temperature and load to illustrate the application in optimization studies. Experimental