Progress in R&D of Molten Salt Electrolytes for Molten Salt Batteries

Kurzfassung

Increasing share of the intermittent renewable energy resources from PV and wind energy in the energy system has led to much progress in research and development in large scale batteries like molten salt batteries (MSBs) [1-6] for grid storage. Compared to other batteries like Li-ion batteries, MSBs (e.g., ZEBRA (Na-NiCl2) battery [2-3] and liquid metal battery (LMB) [4-5]) operate at higher temperatures (generally above 200°C) by using molten salts and liquid metals as active materials, and have the advantages, e.g., longer lifetime, lower costs, larger cell storage capacity. For the commercial ZEBRA battery, replacing Ni with abundant and low-cost Zn makes it more cost effective and could also reduce the operating temperature [6]. In the EU H2020 SOLSTICE project [7], two kinds of Na-ZnCl2 batteries are investigated. One uses the commercial ZEBRA concept [2-3, 9-10], i.e., a solid electrolyte is used in the battery, while another does not use the solid electrolyte to further improve its cost-effectivity and make it possible to have a MWh-scale storage capacity for each cell [8, 11]. In this presentation, progress in research and development of molten salt electrolytes for molten salt batteries (e.g., Na-ZnCl2 batteries [8-11], LMBs [12-13]) will be presented. A simulation-assisted method for molten salt electrolyte selection was developed at DLR. This method is based on simulation of e.g., phase diagrams and vapor pressures of salt mixtures via FactSageTM and thermo-analytical techniques (Differential Scanning Calorimetry (DSC) and OptiMeltTM). It has shown its successful application in development of molten salt batteries, e.g., Na-LMB with the selected LiCl-NaCl-KCl (59:5:36 mol %) by DLR ran stably over 700 cycles at 100 mA cm−2 at 450°C with a coulombic efficiency of 97%, and energy efficiency of about 80% [5]. Acknowledgements This research has been performed by funding of the European Union’s Horizon 2020 research and innovation programme under grant agreement No 963599 (Sodium-Zinc molten salt batteries for low-cost stationary storage, SOLSTICE). References [1] Molten-salt batteries: Pros and Cons of a 40-year-old “Innovation”. https://www.flashbattery.tech/en/molten-salt-batteries-operation-and-limits/ (access on 18th of March, 2025). [2] J. Sudworth, The sodium/nickel chloride (ZEBRA) battery, J. Power Sour., 100 (2001) 149. [3] G. Graeber, et al. Rational cathode design for high-power sodium-metal chloride batteries, Adv. Funct. Mater. 31 (2021) 2106367. [4] H. Kim, et al. Liquid metal batteries: Past, present, and future, Chem. Rev. 113 (2013) 2075. [5] H. Zhou, et al., A sodium liquid metal battery based on the multi-cationic electrolyte for grid energy storage, Ener. Stor. Mater. 50 (2022) 572–579. [6] X. Lu, et al. A novel low-cost sodium–zinc chloride battery. Energy Environ. Sci. 6 (2013) 1837. [7] EU H2020 SOLSTICE project: https://www.solstice-battery.eu/ (access on 18th of March, 2025). [8] J. Xu, et al. Na-Zn liquid metal battery. J. Power Sour., 332 (2016) 274. [9] S. Kumar, W. Ding, et al., AlCl3-NaCl-ZnCl2 Secondary electrolyte in next-generation ZEBRA (Na-ZnCl2) battery. Batteries, 9(8) (2023) 401. [10] L. Sieuw, et al., Influence of precursor morphology and cathode processing on performance and cycle life of sodium-zinc chloride (Na-ZnCl2) battery cells. Ener. Stor. Mater., 64 (2024) 103077. [11] W. Ding, et al., Molten salt electrolyte for Na-ZnCl2 all liquid battery for grid storage. Preprints 2025, 2025020384. https://doi.org/10.20944/preprints202502.0384.v1 [12] Q. Gong, et al., Molten iodide salt electrolyte for low-temperature low-cost sodium-based liquid metal battery. J. Power Sour., 475 (2020) 228674. [13] W. Ding, Q. Gong, et al., Multi-cationic molten salt electrolyte of high-performance sodium liquid metal battery for grid storage. J. Power Sour., 553 (2023) 232254.