Al-Fe Based Molten Salts for Long Duration Energy Storage

There is a need for high energy density long duration energy storage that utilizes earth abundant elements, such as Al and Fe. Energy storage devices that utilize molten salt electrolytes can take advantage of these earth abundant materials and promise to deliver high energy densities. Continued development and in-depth investigation of these fully inorganic, molten salt electrolytes is necessary for the continued progress of low-cost battery technologies. We have been exploring the properties of a new molten salt system which utilizes Al and Fe components and can feasibly be operated as an electrochemically active medium for long duration energy storage. Ternary molten salt compositions of AlCl 3 -FeCl 3 -NaCl and AlCl 3 -FeCl 2 -NaCl were evaluated for their phase behavior and electrochemical performance at intermediate temperatures (~160 - 210 °C). We found that certain compositions of AlCl 3 -FeCl 3 -NaCl were fully or nearly fully molten at 160 °C and that more AlCl 3 was needed in the melt to facilitate the FeCl 2 dissolution. The electrochemistry of these salts was probed and found that the Fe species will cycle between the Fe 3+ /Fe 2+ oxidation states (as FeCl 3 /FeCl 2 ) but that at lower temperatures, the FeCl 2 will precipitate out of the melt and block the electrode. By increasing the test temperatures, the produced FeCl 2 dissolves into the melt allowing the un-blocked electrode to sustain large reduction currents. Future work will investigate ways to increase FeCl 2 concentration in the melt and to facilitate its oxidation in order to optimize the molten salt catholyte electrochemical behavior and demonstrate it in a working battery. This work was supported by the U.S. Department of Energy’s Office of Electricity through the Energy Storage Research Program, managed by Dr. Imre Gyuk. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and 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. SAND2022-16623 A