Printable Ink Development for Molten Salt Battery Electrodes

Abstract

Thermal batteries are important power sources in applications requiring a long shelf life, fast activation, and reliable power delivery. They operate at high temperatures and utilize molten salt electrolytes, typically eutectic mixtures, which are inert solids at ambient temperatures and provide fast ionic conductivity above the melting point of the salt. The size, energy density, and mechanical properties of thermal batteries are limited by the current manufacturing processes, which are slow and labor-intensive. The development of printable, coated-film electrodes and components will lead to batteries with increased energy density, a more efficient production process, and form factor flexibility.The electrolyte salt (LiCl-KCl eutectic) and MgO separator materials are combined in a single ‘electrolyte binder’ mixture (EB). Slurries were prepared with the EB using DMSO as the carrier solvent and additional dissolved LiCl as a binder for the printed film. The slurries show a large change in viscosity as a portion of the electrolyte dissolves during the mixing process. The initial coarse slurry mixture quickly increases in apparent viscosity before smoothing out into highly shear-thinning suspension with a small overall particle size (<25 µm). The slurries can be easily printed directly on coated cathode substrates, using a slot-die or tape casting coating method. After drying, the film provides a stable separator layer, which can be cut or punched into the desired form and assembled into battery cells. Micro-CT imaging shows the resulting multi-layer film with a distinct cathode layer, and separator layers with a final dry film thickness on the order of 220 µm.Thermal analysis data indicates that neither the modified salt content nor the dissolution and re-crystallization process significantly alter the melting behavior of the electrolyte salts and separator coatings when compared to the standard thermal battery separator mixtures. Thermogravimetric analysis (TGA) indicates that the solvent is sufficiently removed from the printed films during the drying process, and differential scanning calorimetry (DSC) measurements show that the melting point of the electrolyte in the coated separator film shifts by less than 5ºC, in comparison to the standard LiCl-KCl eutectic mixture. Single-cell electrochemical tests using the combined multi-layer separator/cathode films yielded voltages of >1.9 V delivering a capacity >100 mAh/g and with a lower internal resistance than standard thermal battery cells.Figure 1. Cross-sectional micro-CT image of printed multi-layer electrode, consisting of carbon fiber support with cathode film and separator/electrolyte overcoat. 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. SAND No. SAND2020-5349 A Figure 1