Enabling Rechargeable Thermal Batteries Using Solid-State Electrolytes
Authors: Herman David , Allcorn Eric , Melad Aaron , Schorr Noah B. , Wesolowski Dan 
Published: December 2024

Thermal batteries are high-temperature power sources designed for devices requiring high power, fast and reliable activation, and long shelf life. They are typically implemented as single-use primary batteries, with a molten-salt electrolyte that allows for high ionic conductivity after melting. However, the common lithium/chalcogenide chemistry (e.g., FeS2) is intrinsically reversible [1]; it is primarily the molten-salt electrolytes which are the barrier to rechargeability, as this design results in irreversible self-discharge due to dissolution of the active materials into the molten salt [2], along with various side reactions. By replacing the molten salt with a solid-state separator, we can enable the design of rechargeable thermal batteries that can operate in thermal environments as high as 500ºC. This approach will allow for the development of high-temperature rechargeable batteries for military, space, and other applications.
Our novel rechargeable thermal battery cell is designed to use FeS2 as the active cathode material and a lithium anode (LAN) in which liquid lithium is immobilized in a secondary matrix of electrochemically inactive material. This LAN is able to provide the electrochemical benefits of metallic lithium, a material that is typically avoided in thermal batteries due to their operation above the lithium melting point. We use tantalum-doped LLZO (garnet-type solid electrolyte) which we show to be compatible with both the anode and cathode materials, even at high operating temperatures. The Ta-LLZO performs well with a high ionic conductivity (compared to typical lower-temperature operation of most rechargeable materials) and provides high-power (and high current density) discharge performance. In comparison to standard thermal battery designs - which lose more than 80% of their capacity after a single attempt to recharge - the Li/LLZO/FeS2 cells can typically be cycled on the order of 100 times while retaining more than 70% of the initial capacity.
[1] T. D. Kaun, M. J. Duoba, K. R. Gillie, M. C. Hash, D. R. Simon, and D. R. Vissers, “Development of a sealed bipolar Li-alloy/FeS2 battery for electric vehicles”, Presented at the 25th Intersociety Energy Conversion Engineering Conference, 1990, pp. 12–17.
[2] P. J. Masset and R. A. Guidotti, “Thermal activated (“thermal”) battery technology. Part I: An overview”, J. Power Sources, vol. 161, pp. 1443-1449, 2006.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Figure 1. Cross-sectional SEM image of a Li/LLZO/FeS2 thermal battery cell after repeated cycling at 450ºC.

Figure 1