Mesoscale Modeling and Simulation of Composition, Manufacturing, and Microstructure Effects on Electrical Conduction in Thermal Battery Cathodes

Li/FeS₂ thermal batteries provide a stable, robust, and reliable power source capable of long-term electrical energy storage without performance degradation. These systems rely on a eutectic salt that melts at elevated temperature, activating the cell. When the electrolyte melts, the cathode becomes a suspension, with cathode particles suspended in a liquid molten salt. This suspension experiences mechanical deformation, or “slumping.” This slump changes the mechanical compression of the cell, as well as the tortuosity and electronic and ionic conductivity of the cell as the mesostructure of the cathode is reordered in response to the external compressive stress. The combined effect of deformation, component composition, and manufacturing conditions on electrical conduction has not been studied, yet the cathode electrical properties are critically important to battery performance.

We present simulation results from a computer model validated with experiments to elucidate the effects of conduction in FeS₂ cathode pellets when composition and manufacturing parameters are varied. Experimental validation was achieved through impedance spectroscopy measurements of pressed-powder cathode pellets. Pellets were manufactured in-house with variations in pellet density, FeS₂ particle size distribution, and ratio of FeS₂ to electrolyte content. Impedance measurements of solid-state pellets were taken at 25, 100, and 200°C before and after slumping. We observed that electrical conductivity increased with relative FeS₂ content and decreased with the FeS₂ particle size distribution, but that the density of the pellets did not demonstrate a clear effect. Slumped pellets exhibited greater electrical conductivity at all temperatures and pellet thickness decreased by 1.8% on average due to slumping.

Sandia’s novel conformal decomposition finite element method (CDFEM) was applied to surface-meshed geometric representations of cathode microstructures generated from microcomputed tomography (MicroCT) reconstructions. MicroCT was performed on all pellet types before and after slumping, with approximately 0.5 μm pixel size achieved for >0.1 mm³ volumes. A particle surface mesh was generated for each volume using FEI Avizo software and combined with the CDFEM background mesh. Results from the SIERRA/Aria finite element code will be presented, including the effects of anisotropy, mesh size, system volume, and manufacturing conditions on electrical conduction through cathode models, and compared with the experimental trends that were observed in the laboratory. The understanding of manufacturing effects on battery performance is not well developed, and this effort represents a step forward in correlated and predicting performance of cells based upon observed manufacturing trends.