Computational Fluid Dynamics Modeling of a Lithium/Thionyl Chloride Battery with Electrolyte Flow

Abstract

This paper is a continuation of the recent series of work to explore computational fluid dynamics (CFD) techniques in conjunction with experimentation for fundamental battery research. The application of interest in this work is a lithium/thionyl chloride primary battery. As a power source, this battery has many desirable characteristics such as high energy and power densities, high operating cell voltage, excellent voltage stability over 95% of the discharge, and a large operating temperature range. As such, there has been a number of one-dimensional modeling studies [1][2][3][4][5][6] in the literature. Modeling efforts by Szpak et al. 1 and Cho 2 focused on the battery's high-rate discharge and the ensuing thermal behavior. Other modeling efforts employed concentrated solution theory and porous electrode theory in the various regions of the cell (e.g., separator, porous cathode) to develop one-dimensional models of the battery. [3][4][5][6] These models examined utilization issues at low to moderate currents, but they differ in how the excess electrolyte was treated.In the lithium/thionyl chloride cell, the solvent is also the reactant, and the volume it occupies is more than that of the reaction products. Therefore, more electrolyte is placed in the cell than can occupy the initial void volume of the separator and porous cathode. The excess electrolyte resides in the head space volume above the active regions. Due to the one-dimensional nature of their models, Tsaur and Pollard 3 and Evans et al. 4 introduced a fictitious reservoir region between the separator and the porous cathode. In these models, the electrolyte fills uniformly from this reservoir region, through the separator, and into the cathode. In contrast, Jain et al. 5,6 modified the one-dimensional equations so that the electrolyte flows from the head space directly into the porous cathode, thus replicating flow in a second, perpendicular direction. An advantage of this latter approach is that the model can predict the drying of the cell due to insufficient electrolyte loading. While these one-dimensional models are simple and efficient at predicting the discharge of a Li/SOCl 2 battery, a multidimensional model fully accounting for the electrolyte flow without making ad hoc approximations is deemed valuable to gain a more fundamental understanding of the processes that occurr in this battery system. The present paper describes such a model and a finite-volume method of CFD to simulate a Li/SOCl 2 battery in the operating regime of low to intermediate discharge rates. The model uses the physical parameters estimated from experimental data 6 to predict discharge curves accurately at various temperatures. Numerical ModelDescription of a Li/SOCl 2 system.-A schematic of a lithium/ thionyl chloride (Li/SOCl 2 ) cell is shown in Fig. 1. The system is identical to the one studied most recently by Jain et al. 5,6 Basically, the cell consists of a lithium foil anode and LiCl film, a separator (glass matting), and a porous carbon cathode support with the ...

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