For the U.S. Department of Defense, high-performance power sources are a critical technology for current and future gun-fired munitions. Molten salt thermal batteries are the reserve power source of choice for many weapon systems due to their high power density, proven long shelf life, and capability to function across a wide range of operating environments. The incorporation of nanomaterials into thermal battery components offers the potential to produce batteries with performance improvements, such as higher voltages at increased current densities. In this study, nanoscale iron disulfide (FeS2) was synthesized using a fully scalable, high energy milling method, and characterized utilizing SEM, XRD, ICP-OES, BET surface area analysis, TGA-DSC, and other techniques. Nanostructured cathode mixtures containing the nanoscale FeS2 and LiCl-KCl eutectic salt electrolyte were pressed into electrode pellets. Their electrochemical performance was then characterized as compared to standard FeS2 cathode electrode pellets currently used in industry. Single cell thermal battery discharge testing, polarization type scans, and pulse load capability were used to compare cell performance for better understanding of best-use applications for these materials. Compared to control cells, the cells utilizing nanostructured cathodes provided higher power response, with respect to both voltage and runtime.