A Mathematical Model of a Lithium/Thionyl Chloride Primary Cell

Abstract

A one-dimensional mathematical model for the lithium/thionyl chloride, primary cell has been developed to investigate methods of improving its performance and safety. The model includes many of the components of a typical lithium/ thionyl chloride cell such as the porous lithium chloride film which forms on the lithium anode surface. The governing equations are formulated from fundamental conservation laws using porous electrode theory and concentrated solution theory. The model is used to predict one-dimensional, time dependent profiles of concentration, porosity, current, and potential as well as cell temperature and voltage. When a certain discharge rate is required, the model can be used to determine the design criteria and operating variables which yield high cell capacities. Model predictions can be used to establish operational and design limits within which the thermal runaway problem, inherent in these cells, can be avoided.The lithium/thionyl chloride (Li/SOC12) cell is an attractive primary energy source because of its high energy density (1, 2). However, researchers have observed that the Li/SOCI~ cell is a serious safety hazard under certain conditions (2). High discharge rates and high temperatures promote thermal runaway which can result in the venting *Electrochemical Society Student Member. **Electrochemical Society Active Member.of toxic gases and explosion. A mathematical model of this battery has been developed to investigate the operational and design characteristics which can be adjusted to yield efficient, yet acceptably safe Li/SOC12 cells.

Material Balance Modification in One‐Dimensional Modeling of Porous Electrodes

“…The superficial current density in the solution, i 2 , is due to the movement of charged species, given as [2] where N i is the average flux density of species i in the pores averaged over the cross-sectional area of the electrode. By conservation of charge, the charge leaving the matrix phase must equal the charge entering the solution phase, and this can be expressed as No header is present, and so the electrolyte level in the electrode drops as a result of volume reduction.…”

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

“…Applying the general species conservation equation in Table I of Wang et al 9 to the species of interest in the Li/SOCl 2 system, one obtains [4] for Li ϩ , and…”

“…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.…”

“…7. Effects of double-layer charging and self-discharge are ignored.Governing equations.-Based on the above-stated assumptions, a two-dimensional model for Li/SOCl 2 batteries can be derived from the general modeling framework previously developed by Wang et al 9 Specifically, the present model consists of equations governing conservation of species, charge, mass and momentum in electrolyte and electrode matrix phases, respectively.Applying the general species conservation equation in Table I of Wang et al 9 to the species of interest in the Li/SOCl 2 system, one obtains [4] for Li ϩ , andfor SOCl 2 . Here, ٌ ϭ ∂/∂x i ϩ ∂/∂y j is for the two-dimensional Cartesian coordinate system, c is the concentration of a species in the electrolyte, and j is the volumetric reaction current resulting in production or consumption of the species.…”