Final report of DFG project: Study on Corrosion Control and Low-Temperature Electrolytes for Low-Cost Na based Liquid Metal Batteries

Abstract

Liquid metal batteries (LMBs) are characterized by an easy scalability and long life, which makes them particularly suitable for large-scale energy storage to stabilise the power grids for the expansion of volatile renewable energies. In this research project, sodium (Na) was investigated as potential negative electrode material, which brings economic and environmental advantages over other LMBs using, for instance, lithium. New challenges related with the higher operating temperature of Na-based LMBs include a potentially high self-discharge of the cell and corrosion issues of its solid components, which may degrade the cell performance and limit its service life. These challenges were tackled during the course of the project and significant progress in battery performance through appropriate selection of materials and cell design was achieved. Prototype Na-LMB cells based on an optimized selection of materials and cell design achieved excellent battery performance. An Ah-scale Na-LMB test cell with LiCl-NaCl-KCl electrolyte and SbBi9 positive electrode ran over 700 cycles at 450 °C and showed a Coulombic efficiency of >97%, an energy efficiency of about 80%, an active material utilization of about 80%, and an estimated lifetime of >15 000 cycles. The calculation based on a 1 MW/5 MWh demo energy storage plant indicates that the estimated Levelized Cost of Storage (LCOS) of the Na-LMB is lower than 0.029 $/kWh, close to that of hydro pumped storage. The acquired know-how on material compatibility will help to improve durability, design, and scaling of this kind of LMB cells. Moreover, the know-how attained for both material compatibility and the purification of molten salts and liquid metals can also be used in other areas where these media are used as heat transfer fluids, thermal energy storage media or electrolytes. The applications here are diverse and include, for example, areas such as nuclear technology, solar thermal power plants, Carnot batteries, the electrification of process heat, and high-temperature electrochemical plating and extraction of metals.