Thermal batteries use a solid electrolyte that is inactive at ambient temperature and can be stored with little to no power loss for long periods (>25 years). However, the battery could be activated instantly (<1s) to produce high power output ranging from watts to kilowatts. The battery activation is achieved by drastically raising the temperature using a carefully designed pyrotechnic heat source which melts the solid electrolyte and initiate the ion conduction and the eventual activation of the battery.¹ The electrolyte properties such as melting point and ionic conductivity are crucial for the energy density and operational lifetime of the battery. A eutectic mixture of different lithium salts is use as electrolyte.²

Thermal batteries usually operate at temperatures higher than 400 ^oC. This high operating temperature requires a complex thermal design of the battery as these batteries are sensitive to moisture and should be hermetically sealed.³ Reducing the melting point of electrolyte has several advantages such as reducing the activation time and reduce the amount of pyrotechnic material needed to activate the battery. Operating at lower temperature also simplify the thermal design of the battery casing.

Potassium hydrogen fluoride (KF-HF), widely used in fluorine production, has a melting point of 278 ^oC and can be used to produce mixtures with lithium salts that has lower melting points^4,5.

In the present work, novel eutectic electrolyte mixtures using 3 different lithium salts and KF–HF with varying compositions were studied. In order to identify the optimum electrolyte composition, several mixtures with different compositions were studied for the thermal stability and ionic conductivity. This study was focused on the identification and development of suitable Li-salt/KF–HF electrolyte composition as an electrolyte material for reducing operating temperature of thermally activated batteries.

Li-salt / KF-HF mixtures was prepared by an infusion process developed to thoroughly mix the two different compounds without putting the sample in danger of thermal decomposition.

Thermal stability of the electrolyte is an important aspect of electrolyte development. Differential scan calorimetry (DSC) was used to test the melting point and stability of electrolyte mixtures with different compositions. It was observed that by introducing the Li-salt into KF-HF the thermal decomposition temperature increased which provide a bigger operating window for electrolyte.

To determine the ionic conductivity of the mixtures, electrochemical impedance spectroscopy (EIS) method was utilized. A new EIS test cell was design and fabricated based on the test cells used in previous studies. The cell can provide accurate results at different temperatures.^6–8

References

1. R. A. Guidotti and P. Masset, J. Power Sources, 161, 1443–1449 (2006).
2. P. Masset and R. A. Guidotti, J. Power Sources, 164, 397–414 (2007).
3. P. B. Davis and C. S. Winchester, Limiting factors to advancing thermal battery technology for naval applications, NAVAL SURFACE WARFARE CENTER SILVER SPRING MD, (1991) http://www.dtic.mil/docs/citations/ADA247773.
4. R. J. Ring, D. Royston, Australian Atomic Energy Commission, and Research Establishment, A review of fluorine cells and fluorine production facilities, Australian Atomic Energy Commission, Research Establishment, Lucas Heights [N.S.W., (1973).
5. L. M. Dennis, J. M. Veeder, and E. G. Rochow, J. Am. Chem. Soc., 53, 3263–3269 (1931).
6. E. W. Yim and M. Feinleib, J. Electrochem. Soc., 104, 622–626 (1957).
7. E. W. Yim and M. Feinleib, J. Electrochem. Soc., 104, 626–630 (1957).
8. T. Humplik et al., Ionic Permeability within Thermally-Activated Batteries., Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States), (2015) https://www.osti.gov/scitech/servlets/purl/1254704.