Ionic Permeability within Thermally-Activated Batteries

Thermally-activated (or molten salt) batteries utilize high melting temperature (> 300 ºC) salts as the electrolyte. Since the electrolyte is solid at room temperature, an advantage of these types of batteries is that there is no self-discharge prior to activation, which significantly increases the shelf life compared to conventional electrochemical batteries. To activate the battery, a heat source (usually an internal pyrotechnic) is required to melt the electrolyte and enable current flow. During the melting/activation process, the separator in each cell thins until the oxide binder supports the cell compaction force. An open question is what becomes of the melted electrolyte within the separator and electrodes during activation. The transport and infiltration of the electrolyte into the electrodes can reduce the internal impedance of the cell; however, excess electrolyte could cause shortage and failure of the battery. Therefore, an improved fundamental understanding of the governing transport properties of the electrolyte is needed to better predict the performance of thermal batteries during activation.

In this work, we performed a series of fundamental studies to probe the ionic mobility and conductivity of the electrolyte.  First, by utilizing a series of single cells frozen at various activated times, we performed electron probe microanalysis (EPMA) on the cell cross-sections to quantify the transport of bromine tracers. Through the use of image analysis techniques, we mapped out of the concentration of bromine throughout each cell and elucidated the complex process of electrolyte diffusion and flow. Next, using a custom-built pressure vessel cell, we quantified the permeability of the electrolyte through the oxide binder during and after activation. Furthermore, we investigated the effects of compaction pressure on both the permeability and internal impedance of the separator. Collectively, these experimental studies provide quantitative transport information about the electrolyte, which can be used to inform predictive models for battery activation and improve the understanding of the impact of separator compaction on electrolyte transport properties. 

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.