Study of Lithium-Silicon Anode Materials for Thermal Batteries

Thermal batteries are constructed from a molten salt electrolyte which is solid at room temperature, a FeS 2 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 FeS 2 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 Li 13 Si 4 as the negative electrode. Lithium silicides were made using solid state synthesis in a tube furnace. The electrochemical tests show that the Li 13 Si 4 is a better negative electrode material than Li 7 Si 3 (Figure 1). In addition, powder neutron diffraction was used to probe the lithium silicides with increasing temperature. The high-temperature behaviour of Li 13 Si 4 and Li 7 Si 3 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 Li 13 Si 4 and Li 7 Si 3 . The results indicate that Li 13 Si 4 and Li 7 Si 3 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