Electrochemical Parameters of Na-based Liquid Metal Batteries

Low-cost sodium-based liquid metal batteries are attractive candidates for grid-scale stationary energy storage. In this presentation, Na//SbBi9 test cells with the molten salt electrolyte LiCl-NaCl-KCl (61-3-36 mol%) are introduced and their electrochemical performance is presented. Two different cell designs were realized, namely cells with a metal foam hosting the negative electrode (5-6 Ah nominal capacity) and cells without foam. All batteries were operated at around 450 °C in an all-liquid state, i.e., both electrodes and the electrolyte remained liquid during operation. When cycled at 100 mA/cm², a Coulombic efficiency typically >98% and an energy efficiency around 70% was achieved [1]. From the charge-discharge cycling tests, the self-discharge current and the ohmic resistance of the cells could be determined as a function of the electrode distance. The cell voltage in dependence on the cell’s state-of-charge was obtained from intermittent charging/discharging tests.
Besides the described basic characterization of the electrochemical cell performance, this presentation focusses on the buildup of the different overpotentials in an operating all-liquid Na-LMB. ... mehrFor this purpose, the intermittent charging/discharging tests and electrochemical impedance spectroscopy (EIS) data were analyzed in detail. Model fitting allowed the identification and quantification of charge-transfer processes as well as of the diffusive and convective mass transport of Na inside the positive electrode [2]. The observed convergence of the mass-transport overpotential in the long-term/low-frequency limit is attributed to thermal convection in both the electrolyte and the positive electrode. When applying large enough currents, an asymmetry between charging and discharging was observed, which is explained by an additional solutal effect influencing the convection in the positive electrode.