Integration of Low Melting Temperature Electrolyte in Thermal Batteries
Authors: Yazdani Aliakbar , Sanghadasa Mohan , Botte Gerardine G. 
Published: December 2020

Thermal batteries usually operate at temperatures higher than 400 °C. This high operating temperature requires a complex thermal and mechanical design of the battery components and hermetically sealed casing. Reduced operating temperature thermal cells can benefit from the majority of intrinsic characterizations of thermal batteries such as safety, robustness, and infinite shelf life while mitigating the challenges related to the high operating temperature batteries such as exotic thermal insulation materials, complicated internal pyrotechnic heat source, and heavy packaging. To integrate the thermal batteries into a wider range of applications such as bore-hole probes, smart grid energy storage, and geothermal energy applications, operating temperatures of these batteries should be reduced.

The most common design for thermal batteries is pellet stacks. The pellet pressing fabrication method inherently causes issues due to the fragility of the pellets, making handling and assembly of the cells arduous. Additionally, due to the liquid nature of molten salt electrolyte used in these batteries, a structure to immobilize the electrolyte layer during the operation of the battery is required. The role of the immobilizing component is also to prevent short circuiting while the battery is affected by severe physical shock or vibration. The most commonly used binder material in today’s thermal batteries is magnesium oxide (MgO). Extensive investigations were conducted on the properties and the electrochemical effect of using MgO as the binding material, besides, its fabrication process is well optimized. However, using MgO is not without its drawbacks. To prevent short-circuiting during the cell operation especially in the presence of an external shock to the system, approximately 30 vol.% of the separator layer should be dedicated to MgO, resulting in lower than desired energy density. Ceramic felts showed promising results in recent studies. High level of porosity (e.g. 96% bulk porosity), outstanding stability at elevated temperatures, and mechanical stiffness could be the key to make batteries that can operate for a longer time.

Our research group has previously developed a novel eutectic electrolyte mixture using potassium bifluoride and different lithium salts such as lithium fluoride as the low melting point electrolytes. In this research, initially, the integration of this electrolyte mixture in a thermal cell was tested. Consequently, aluminum oxide (Al2O3) and yttria-stabilized zirconia (YSZ) ceramic felts were loaded with the low melting electrolyte as an alternative to the current pellet-based separator layer. Finally, (i) the assembly of the cell, (ii) the thermal analysis of the components by differential scan calorimetry (DSC) method, (iii) the new electrolyte/ceramic-felt combination’s loading level, wetting properties, infiltration, and leakage, and (iv) electrochemical behavior of the new thermal battery such as discharge capability and energy density were investigated.