Novel Thermal Batteries Based on Sodium Chemistries

Abstract

Thermal batteries are high temperature primary batteries that function when an electrolyte, that is solid at room temperature, becomes molten at the operating temperature of the battery between 350 and 550 °C. This type of battery is valuable in high consequence scenarios when high power is required rapidly such as in ejector seats and space flight. Thermal batteries can withstand very harsh treatments and can be stored for many years without self-discharge. The most common thermal battery used today consists of an Li13Si4 anode, LiCl-KCl electrolyte and an FeS2 or CoS2 cathode. However, rising concerns regarding the sustainability of lithium has required that alternative chemistries such as using sodium be explored.The use of sodium as an anode on its own is not suitable in thermal batteries due to its low melting point of 100 °C, so the use of sodium-based alloys such as NaSn (melting point 578 °C) and Na3Sb (melting point 859 °C) is investigated in this work. The identification of electrolytes with suitable melting points that are compatible with the operating temperature of the thermal battery and the melting point of the anode has also been undertaken since the electrolyte needs to melt at least 100 °C lower than the anode. Three such eutectic salts are NaCl-MgCl2 (melting point 451 °C), NaCl-MgCl2-KCl (melting point 383 °C) and NaCl-MgCl2-CaCl2 (melting point 424 °C).Solid state and mechano-synthesis methods for preparation of the anode and electrolyte materials will be discussed as well as characterisation techniques such as PXRD, SEM, EDS, impedance measurements and DSC for the individual components of the battery. The impedance measurements are used to evaluate the conductivity of both the electrolyte and anode materials. The conductivity of the sodium-based electrolytes has been found to have the same order of magnitude as the LiCl-KCl electrolyte used in the standard thermal battery, typically between 0.5 and 2 Scm-1. Assembly and testing of a thermal battery cell will also be elucidated. Preliminary tests of a sodium based thermal battery with an FeS2 cathode have given voltages up to 2 V.