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, cata- strophic failure, especially when operated at high discharge rates. High discharge rates and high temperatures promote thermal run- away, 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 funda- mental 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 solu- tion 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 elec- trode. 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 parame- ters as a function of temperature prevented quantitative comparisons. Evans and White7 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 dis- charged 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 bal- ance in the porous cathode that accounts for electrolyte replenish- ment 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/cm2 for a D-size cell). Previous thermal models of Li/SOCl2 cells have shown that under these operating con- ditions 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 A one-dimensional mathematical model of a spirally wound lithium/thionyl chloride primary battery is developed and used for parameter estimation and design studies. The model formulation is based on the fundamental conservation laws using porous elec- trode theory and concentrated solution theory. The model is used to estimate the transference number, the diffusion coefficient, and the kinetic parameters for the reactions at the anode and the cathode as a function of temperature. These parameters are obtain ed by fitting the simulated capacity and average cell voltage to experimental data over a wide range of temperatures ( 255 to 498C) and discharge loads (10-250 V). The experiments were performed on D-sized, cathode-limited, spirally wound lithium/thionyl chloride cells. The model is also used to study the effect of cathode thickness on the cell capacity as a function of temperatu re, and it was found that the optimum thickness for the cathode-limited design is temperature and load dependent.