Electrical Conductivity Measurements of the Binary NaCl-CaCl2, the Ternary NaCl-CaCl2-BaCl2 and the Quaternary NaCl-CaCl2-BaCl2-ZnCl2 Molten Salts

Abstract

An emerging set of batteries, called the molten salts or all liquid battery, are designed for cheap energy storage and electrical grid buffering purposes (1). Some molten salt batteries are commercial or close to commercial like the ZEBRA, NaS and Ambri batteries, however, most of these battery technologies are still on an experimental, lower TRL level but can become increasingly important in the future as the shift towards more intermittent energy sources emerge and as the development of these batteries become more mature. One suggested battery technology is the Na-Zn battery (2). This battery technology is currently further investigated in the EU funded SOLSTICE project (Grant Agreement No 963599). The Na-Zn battery will have Na and Zn electrodes and the electrolyte – the molten salt – will be a NaCl-CaCl2 based electrolyte, with additions of BaCl2 or perhaps even BaCl2 and SrCl2. The NaCl content of the electrolyte gives the charge of the battery, CaCl2 lowers the operating temperature and BaCl2 and SrCl2 increases the density of the electrolyte. The operating temperature of the battery is approximately 600 °C and joule heat generated inside battery during charging and discharging due to resistance of the electrolyte allows to maintain this operating temperature. The composition of electrolyte changes with the battery state of charge hence the internal resistance of battery is not constant during normal operation. Therefore, precise data of electrical conductivity of the electrolyte is required for modelling its performance thermal balance.A method for measuring electrical conductivity has been built in the SINTEF laboratory. The setup used for the measurements is presented in Figure 1a. It consists of two tungsten electrodes immersed in molten electrolyte – one electrode is stationary and the other is moving inside a capillary. Position of the moving electrode inside the capillary is precisely controlled by a height gauge. Shielded thermocouple is used for measuring temperature. To avoid polarization of electrodes during measurement, electrical impedance spectroscopy (EIS) at open circuit potential is used. The principle of the method is that resistance between the electrodes is measured at different positions of the moving electrode inside the capillary. Measured resistance can be expressed by the following formula: R=R0+ρ·(l/A) Where R0 is the offset resistance of electrolyte outside capillary including resistance of electrodes and leads,ρ is the resistivity of electrolyte, l is the length between end of capillary and tip of the moving electrode, A is the area of the capillary. The measurement of resistance is repeated at different positions of the moving electrode relative to the chosen reference position. Since R0 and A are constant, resistivity can be calculated knowing only the tangent of function R = f(l) (Figure 1b) divided by A. The presented approach is characterized by high precision and robustness due to no requirement of cell constant values, which is problematic to estimate at high temperature. It is only based on relative movement of one electrode which is easy to control. In addition, a higher number of positions of the moving electrode at which the measurement is performed effectively helps to eliminate errors. Conductivity is the inverse resistivity. Electrical conductivity measurements were done at a range of temperatures, from 600 °C to 800 °C with 50 °C steps and the results are presented in the form of linear formulas intended for use in User Defined Functions in modelling.Conductivity of various chloride salt compositions containing NaCl, CaCl2, BaCl2 and ZnCl2 have been investigated using the presented technique. It is generally found that the electrical conductivity is almost linear with temperature increase in the range of liquidus temperature up to approximately 800 °C for the investigated electrolyte compositions. Above that region the relationship is not linear anymore. This region is not presented in this presentation. It is also found that the conductivity increases with increasing NaCl content and that larger cations like Ca and Ba reduce the conductivity of the molten salt. The method has proven very powerful to measure electrical conductivity on molten salts and the results are comparable to experimental results found in literature (3).