Characterization of electrochemical processes in Na-based liquid metal batteries on different timescales

The performance of liquid metal batteries (LMBs) is typically characterized by charge-discharging cycling. To gain fundamental understanding of the underlying electrochemical processes, this paper investigates in detail the buildup of the different overpotentials in an operating LMB. As model system, Na/LiCl-NaCl-KCl/Sb-Bi cells are
assembled and tested. Cells with metal foam hosting the liquid Na are compared with foamless cells utilizing an insulating sheath. Based on data obtained by intermittent charging/discharging tests and electrochemical impedance spectroscopy, charge-transfer processes as well as diffusive and convective mass transport of Na inside the positive electrode are identified and quantified as function of the cell's state-of-charge. The conver-gence of the overpotential in the long-term/low-frequency limit is attributed to thermal convection if low currents are applied (impedance measurements). When applying larger currents such as the ones used for intermittent charging/discharging, an additional contribution attributed to solutal convection is observed and quantified. Thus, all processes relevant for the electrochemical performance of the tested Na-based LMB cells are captured, completely spanning the range from the millisecond timescale up to 1000 s.