Thermal batteries are constructed from a molten salt electrolyte which is solid at room temperature, a FeS2 positive electrode and a lithium-silicon negative electrode. The typical operating temperature of the battery is between 350 to 500°C. Thermal batteries use an inorganic salt as the electrolyte such as binary or ternary alkali halide eutectics. For example, LiCl-KCl and LiF-LiCl-LiBr. At a composition of 44.8 LiCl and 55.2 KCl wt%, the melting point is around 353°C for LiCl-KCl. Molten salts have a very high electrical conductivity, so thermal batteries have a higher power density than room temperature batteries. In this work, LiCl-KCl eutectic was used for electrochemical testing. The performance of an electrode can be studied in single-cell discharge tests where a discharge profile is obtained. The tests were carried out in a cell testing rig inside an argon glovebox. The cells were made from pellets which were placed in a Swagelok casing. The cells were made from 0.15g of FeS2 mixed with 0.05g LiCl-KCl as the positive electrode, 0.2g of LiCl-KCl with 35% wt MgO as the electrolyte and 0.2g of Li13Si4 as the negative electrode. Lithium silicides were made using solid state synthesis in a tube furnace. The electrochemical tests show that the Li13Si4 is a better negative electrode material than Li7Si3 (Figure 1). In addition, powder neutron diffraction was used to probe the lithium silicides with increasing temperature. The high-temperature behaviour of Li13Si4 and Li7Si3 was studied in March 2020 by the JTSI group at the ISIS neutron source, Rutherford Appleton Laboratory, UK. The data gives information on structural changes in Li13Si4 and Li7Si3. The results indicate that Li13Si4 and Li7Si3 remain remarkably stable at 500°C. There are no phase transitions in the materials from room temperature to 500°C and the materials remain crystalline. These findings give a clearer understanding of thermal battery discharge processes. Figure 1