Development of halogen salts as low-temperature electrolytes in Na-based liquid metal batteries for low-cost large-scale electricity storage

Kurzfassung

The decarbonization of the energy intensive industry branches can only be achieved by the implementation of scalable, inexpensive storage solutions which finally allow for the generation of dispatchable electricity. Batteries play a substantial role in that context but have yet been too expensive to be solutions in the GWh-range. The development of liquid metal batteries is one of the ground-breaking developments in the field since it offers high storage capacity and long life-time at overall moderate costs. Yet, liquid electrolytes are required which allow for the implementation of sodium-based liquid metal batteries (Na-LMBs) with low operating temperatures, long life-times and reduced self-discharge phenomena. The high melting point of electrolytes of Na-LMBs leads to high demands on corrosion protection while high sodium solubility results in high self-discharge and thus affects the energy efficiency of the battery. In this thesis, Li-K-Na//X (X=Cl and I) molten salts were investigated to identify salt compositions with low melting point and low sodium solubility to be used as electrolytes for Na-LMBs. Firstly, simulations were carried out using FactSage to pinpoint eutectic compositions based on halide molten salts. Subsequently, the melting behavior of the most promising molten salts was analyzed using differential scanning calorimeter (DSC) and confirmed in melting point apparatus named OptiMelt. By the means of simulations and experiments, Li-K-Na//X ternary systems were incorporated into a pseudo-binary system, in which finite NaX dissolves in a eutectic LiX-KX binary system. We demonstrate that increasing concentrations of NaX only impose minor changes in the melting point (±5 °C) of the quasi-binary Na,K,Li//X system. More specifically, a total of 9% Na+ can be dissolved in the LiCl-KCl system resulting in a eutectic composition of 53.9 : 37.1 : 9 mol% (Li:K:Na) with melting point of 352°C. In the iodide-based salt up to 7 mol% Na+ can be dissolved resulting in a eutectic composition of 59.2 : 33.8 : 7 mol% (Li:K:Na) and a melting point of 288.9 °C. Li-KNa//X molten salts are promising candidates of electrolytes for low operating temperature and high energy efficiency Na-LMBs.